Written Description

The Written Description Requirement under US Patent Law

The written description requirement under 35 U.S.C. §112(a) plays a foundational role in U.S. patent law. This requirement mandates that a patent specification contain a "written description of the invention" in a full and clear manner. Over decades, this statutory provision has evolved through jurisprudence to function as a gatekeeping doctrine that ensures an inventor was in "possession of the claimed invention" as of the filing date. This article surveys the written description requirement, including its statutory foundation, major case law, how it is applied by the courts and United States Patent and Trademark Office, and practical tips for satisfying the requirement.

Statutory Foundation of the Written Description Requirement

The written description requirement is codified in the first paragraph of 35 U.S.C. § 112(a), which provides that the application must contain a written description of the invention, including the process of making and using it, in a full, clear, and concise manner as to enable any person skilled in the art to determine that the applicant was in possession of the claimed invention as of the filing date.

This language has been consistently interpreted by the courts to impose two distinct but related requirements: (1) a written description of the invention, and (2) enablement. The first clause—“a written description of the invention”—is not merely a preamble to the enablement requirement, but an independent obligation. The Federal Circuit reaffirmed this principle in Ariad Pharmaceuticals, Inc. v. Eli Lilly & Co., 598 F.3d 1336, 1344 (Fed. Cir. 2010), where the court held: “Section 112, para. 1, contains a written description requirement separate from the enablement requirement.”

In Ariad, the court emphasized that the statutory phrase “written description of the invention” refers to a disclosure that clearly allows a person of ordinary skill in the art to conclude that the inventor was in possession of the claimed invention as of the filing date. This requirement ensures that the claims define what the applicant actually invented, and are not speculative or based on an after-the-fact realization.

The statute is thus interpreted to require that the specification “conveys with reasonable clarity to those skilled in the art that, as of the filing date, the inventor was in possession of the invention,” as later claimed. The inquiry is factual in nature, assessed on a case-by-case basis, and varies depending on the complexity and predictability of the field of invention.

Written Description Requirement vs. Enablement Requirement

The written description requirement and the enablement requirement are distinct yet related obligations under 35 U.S.C. § 112(a). While both serve to ensure that the inventor has sufficiently disclosed their invention, they address different questions.

The written description requirement asks: Did the inventor actually invent the claimed subject matter as of the filing date? It focuses on demonstrating that the inventor had possession of the claimed invention and conveyed it with sufficient detail in the specification. In contrast, the enablement requirement asks: Can a person of ordinary skill in the art (POSITA) make and use the claimed invention without undue experimentation based on the disclosure?

For example, in Ariad v. Eli Lilly, the claims were directed to methods of reducing NF-κB activity. Although the specification described the desired biological effect and hypothesized mechanisms, it failed to describe any actual molecules or working examples that achieved the disclosed function. The court found a lack of written description because the inventors had not yet conceived how to perform the invention.

In contrast, a claim might satisfy the written description requirement but fail enablement if the invention is clearly possessed but the disclosure does not teach a POSITA how to practice the invention without "undue experimentation" (extensive research). For instance, where the applicant describes a complex genetic construct with no guidance on how to synthesize or deliver they may show possession but not enablement.

Thus, written description ensures the inventor had the idea, while enablement ensures the public can practice it. Both are required for valid claims.

Possession of the Claimed Subject Matter

At the core of the written description requirement is a fundamental and fact-intensive inquiry: Did the inventor possess the claimed subject matter as of the filing date? This inquiry is rooted in the statutory mandate of 35 U.S.C. §112(a), which requires that the specification contain a “written description of the invention.” The Federal Circuit has repeatedly emphasized that the purpose of this requirement is to ensure that the inventor had fully conceptualized and was in possession of the claimed invention when the application was filed—not merely an unformed idea or a research plan with a hoped-for result.

This possession standard is not satisfied merely by describing an end goal or a broad concept. Rather, the specification must convey with reasonable clarity to a person of ordinary skill in the art (POSITA) that the inventor had actually invented what is being claimed. This is evaluated from the perspective of the POSITA, taking into account the nature and predictability of the technology, and the level of detail required to reflect possession will vary accordingly.

The question of whether there is adequate written description is a fundamental factual inquiry that takes into consideration the entirety of the application, including the specification, drawings, and claims to determine whether there is sufficient evidence that the invention was captured in the original filing on the filing date sought. Possession may be demonstrated in a variety of ways, depending on the context and subject matter. The Federal Circuit has identified several illustrative examples of how written description may be satisfied:

Disclosure of Structural Chemical Formulas

In chemical and biotech cases, where claims are directed to compounds or molecules, the inclusion of precise structural chemical formulas may provide conclusive evidence of possession. For instance, in Amgen Inc. v. Chugai Pharm. Co., 927 F.2d 1200 (Fed. Cir. 1991), the Federal Circuit stated that a claim to a specific DNA sequence lacked written description where the specification failed to disclose the sequence itself or any means to predict it.

The Federal Circuit considered whether certain claims in Amgen’s ‘008 patent, covering DNA sequences encoding erythropoietin (EPO), were valid under the written description requirement of 35 U.S.C. § 112. The dispute centered on whether the description in the application relied on by Amgen to demonstrate possession of the full scope of DNA claims, particularly those generically claiming any sequence encoding a polypeptide “sufficiently duplicative” of EPO to exhibit its biological activity, sufficiently supported the claims.

The Federal Circuit invalidated claims 7, 8, 23–27, and 29 for lack of written description. These claims drawn to generic DNA sequences were not sufficiently supported by the disclosure, which described only a limited number of EPO analogs. Although the specification asserted that numerous analogs could be created, the court found that Amgen had provided insufficient examples and technical guidance for producing DNA sequences other than the few disclosed. The court emphasized that the claims require a broad genus of DNA sequences, yet the specification only described and enabled a small subset.

Moreover, the court rejected the notion that the original claim language alone could support the expansive scope of these claims. The court reiterated that a claim's breadth must reasonably correlate with the scope of the enabling and descriptive disclosure. Because Amgen's application did not provide support for the full genus of claimed DNA sequences, the claims were found invalid under § 112(a).

Description of Distinguishing Characteristics

Possession may also be shown by describing features or properties that allow a POSITA to recognize the claimed invention. This is especially relevant where an invention involves biological materials or genera of compounds. In Enzo Biochem, Inc. v. Gen-Probe, Inc., 323 F.3d 956 (Fed. Cir. 2002), the court acknowledged that a deposit of biological material, or a description of identifying characteristics, may suffice if it allows one to distinguish the claimed invention from others.

In Enzo Biochem, Inc. v. Gen-Probe Inc., the Federal Circuit addressed whether the disclosure in Enzo’s patent satisfied the written description requirement of 35 U.S.C. § 112 for nucleic acid probes specific for Neisseria gonorrhoeae. The case focused on whether descriptions in the specification, including references to deposited DNA sequences and their hybridization properties, adequately described the full scope of the claims.

The district court had granted summary judgment invalidating the claims, reasoning that they defined compositions based only on function, preferential hybridization to N. gonorrhoeae over N. meningitidis, and failed to describe the actual nucleotide sequences. However, the Federal Circuit reversed, holding that referencing a deposit in a public depository may be sufficient to satisfy the written description requirement when structural disclosure is otherwise unavailable. The court emphasized that deposits made the biological material accessible to the public and thus potentially descriptive.

To determine sufficiency, the court adopted the USPTO’s written description guidelines, which allow compliance through disclosure of sufficient details of the invention, such as structure, function when correlated to structure, or deposit. Although the exact terms of the sequences were not recited in the specification, the accessibility of the deposited material could establish disclosure.

The court held that if an applicant shows through deposit, function, and expert testimony that one skilled in the art could recognize the claimed sequences, the written description requirement may be met. Because factual questions remained, such as whether the described functional characteristics and deposited sequences were representative, the court remanded for further proceedings, acknowledging that presenting evidence on these issues could prove dispositive.

Actual Reduction to Practice

A description of how the invention was actually made and tested, including experimental results, can strongly support a finding of possession. This type of disclosure shows that the inventor did not merely theorize about the invention but actually implemented it in a working embodiment.

Disclosure of Functional Language, Where Supported by Sufficient Detail

While functional claiming is permissible, merely reciting the function of an invention is not enough. The specification must include sufficient detail, such as representative species, examples, or structural information, to demonstrate that the inventor invented the means of achieving the function. In Juno Therapeutics v. Kite Pharma, 10 F.4th 1330 (Fed. Cir. 2021), the court invalidated claims to a genus of antibodies defined only by function, holding that the absence of representative examples rendered the disclosure inadequate.

The Federal Circuit reversed a jury verdict upholding the validity of claims in U.S. Patent No. 7,446,190, finding that the patent failed to provide an adequate written description of the claimed invention. The asserted claims were directed to a chimeric antigen receptor (CAR) comprising a CD3-zeta signaling domain, a CD28 costimulatory domain, and a single-chain variable fragment (scFv) binding element capable of targeting selected antigens, including CD19.

The Federal Circuit found that the claimed invention encompassed a functionally defined genus of scFvs, those that bind to a "selected target". However, the specification disclosed only two scFv examples, one binding to CD19 and one to PSMA, without amino acid sequences or other structural or functional details that would enable a person skilled in the art to identify which scFv perform the claimed function. The court emphasized that in cases involving genus claims defined by function, the specification must include either a representative number of species or structural features common to the genus members. Here, the patent specification did neither.

Testimony that scFvs were generally known in the art was insufficient to establish possession of the entire genus. The court concluded that the written description failed to demonstrate that the inventors possessed the full scope of the functional genus they claimed, particularly in light of the vast number of possible scFvs and the unpredictability of binding functionality. Accordingly, the claims were held invalid for lack of an adequate written description of the claimed invention.

Adequate Written Description for New or Amended Claims

Under both 35 U.S.C. § 112(a) and 35 U.S.C. § 132(a), an amended claim or a new or amended claim introduced during prosecution must be adequately supported by the originally filed specification. These provisions collectively prohibit the introduction of new matter, that is, subject matter not disclosed in the application as originally filed. If an applicant attempts to amend a claim to recite limitations not supported by the original disclosure, the U.S. Patent and Trademark Office (USPTO) may reject the claim for failing to comply with the written description requirement of § 112.

In In re Rasmussen, 650 F.2d 1212 (CCPA 1981), the Court of Customs and Patent Appeals addressed this very issue. There, the examiner rejected an amended claim under § 132 for improperly introducing new matter. However, the court held that the correct basis for such a rejection is § 112, not § 132. Section 132 prohibits the introduction of new matter procedurally, but the substantive inquiry into whether a claim is supported by the disclosure must be made under § 112. The court emphasized that a claim amendment must be grounded in the original specification and that failing to do so renders the claim unpatentable for lack of written description support.

Amendments to patent claims can become necessary to advance a patent application to a patent. For example, there may be prior art that renders the originally claimed invention obvious. When an applicant amends claims, especially to overcome prior art, the key question becomes whether the claim language finds support in the originally filed specification. If not, the amendment may fail for lack of adequate disclosure, even if the newly introduced subject matter would otherwise be patentable.

Functional Genus Claims and Representative Species Requirement

When an applicant claims a genus defined by functional language, the Federal Circuit has consistently held that the written description requirement under 35 U.S.C. § 112(a) imposes a heightened obligation to demonstrate possession of the full scope of the claimed subject matter. Specifically, the applicant must either (1) disclose a representative number of species falling within the genus, or (2) identify common structural features shared by members of the genus such that a person of ordinary skill in the art (POSITA) can visualize or recognize the scope of the claimed genus.

This principle reflects the concern that functional language, while potentially useful for claiming a broad invention, may overreach the actual contribution disclosed in the specification. Without sufficient structural guidance or exemplification, such claims may cover more than what the inventor actually invented and disclosed as of the filing date, violating the core tenet that patent protection is available only for that which the inventor possessed at the time of filing.

This doctrine was central in Amgen Inc. v. Sanofi, 872 F.3d 1367 (Fed. Cir. 2017), which involved claims to a genus of monoclonal antibodies that bind to a specific epitope of the PCSK9 protein. The court found that although the patent described a few specific antibodies, the broad functional genus claim encompassed potentially millions of other antibodies. Because the patent failed to disclose a representative number of species or structural features sufficient to define the full scope of the genus, the claims lacked adequate written description support.

A similar result occurred in Juno Therapeutics, Inc. v. Kite Pharma, where the patentee claimed a chimeric antigen receptor (CAR) incorporating a single-chain variable fragment (scFv) that binds to CD19. The claims were defined functionally—by binding capability—but the patent did not disclose any actual CD19-specific scFv sequences or sufficient structural detail to represent the genus. The Federal Circuit concluded that the patent did not demonstrate that the inventors possessed the claimed genus and invalidated the claims for lack of written description.

These cases highlight the danger of overbroad functional claiming in unpredictable fields like biotechnology. To satisfy the written description requirement, an applicant must show more than a desired function or result. They must either exemplify a meaningful number of species within the genus or articulate structural characteristics common to the genus members. Absent such disclosure, the claims may be rejected or invalidated as lacking sufficient written description.

Judicial Interpretation and the Role of the Federal Circuit

The Federal Circuit has played a central role in shaping and enforcing the modern contours of the written description requirement, particularly through a series of landmark decisions that have elevated the doctrine into a significant and sometimes controversial gatekeeper of patent validity. Prior to the Federal Circuit’s decisive intervention, the boundary between written description and enablement was often blurred, with courts and practitioners treating them as interrelated inquiries under a single statutory clause. However, this began to change with cases like Regents of the University of California v. Eli Lilly & Co., and culminated with the Federal Circuit’s en banc decision in Ariad Pharmaceuticals, Inc. v. Eli Lilly.

In Ariad, the Federal Circuit squarely addressed whether § 112(a) includes a written description requirement independent from the enablement requirement. The court answered unequivocally in the affirmative, holding that “[t]he specification must describe an invention understandable to that skilled artisan and show that the inventor actually invented the invention claimed.” This doctrinal clarification marked a pivotal shift in patent law. Post-Ariad, the Federal Circuit has consistently enforced written description as a distinct statutory requirement, often applying it stringently, particularly in biotechnology, pharmaceuticals, and other unpredictable arts.

However, this aggressive application has not been without criticism. In his article, “The Federal Circuit’s En Banc Written Description Requirement: Time for the Supreme Court to Reverse Again,” Allen Yu contends that the written description requirement has become an ad hoc safety valve used by courts to invalidate claims they view as too broad or speculative, particularly in cases involving functional claiming or insufficient exemplification. Yu argues that rather than reflecting a coherent or principled doctrinal framework, the written description requirement has evolved into a policy tool that allows courts to avoid the more complex inquiries posed by enablement, such as the undue experimentation analysis.

Critics like Yu caution that this approach undermines the predictability and integrity of the patent system, allowing courts to impose unarticulated standards that may chill innovation, particularly in rapidly developing fields. Nonetheless, the Federal Circuit continues to treat written description as a distinct and essential safeguard that ensures the claims are commensurate with what the inventor actually disclosed and possessed as of the filing date.

USPTO Guidelines and Examination Standards for Determining Compliance

The United States Patent and Trademark Office (USPTO) provides guidance to patent examiners on the application of the written description requirement under 35 U.S.C. § 112(a) through the Manual of Patent Examining Procedure (MPEP) § 2163. These guidelines reflect the USPTO’s interpretation of governing case law, particularly Federal Circuit precedent, and instruct examiners on how to analyze whether a patent application complies with the statutory requirement that the specification “contain a written description of the invention.”

The primary inquiry under MPEP § 2163 is whether the specification as originally filed reasonably conveys to a person of ordinary skill in the art (POSITA) that the inventor had possession of the claimed invention as of the filing date. To make this determination, the examiner must conduct a fact-specific analysis, grounded in the disclosure, the claim language, and the level of skill in the art.

The examiner is directed to assess whether the applicant has described the invention with sufficient detail to enable a POSITA to recognize that the inventor had conceived of the full scope of the claimed subject matter. This includes an evaluation of whether each claim limitation is explicitly or implicitly supported in the originally filed specification. Importantly, support does not require a verbatim match; instead, the specification must include adequate disclosure, through words, drawings, figures, or formulas, that reasonably conveys the inventor’s possession of the claimed features.

Where claims have been amended during prosecution, or where the applicant introduces new or amended claims, examiners must also determine whether these claims are reasonably supported by the originally filed disclosure. If not, the claim may constitute new matter, which is impermissible under § 132, and more substantively, the claim may fail the written description requirement under § 112(a). As clarified in In re Rasmussen, the appropriate ground for rejecting such a claim is § 112, not § 132, because the issue is one of insufficient support, not mere procedural defect.

The guidelines list several factors examiners should consider when determining compliance with the written description requirement. These include the predictability of the art, the breadth and nature of the claims, the level of detail provided in the disclosure, and whether the specification includes representative examples or structural features for any claimed genus. For example, in unpredictable fields like biotechnology, mere functional descriptions may be insufficient unless accompanied by representative species or clearly defined structural characteristics.

A written description rejection is appropriate when the examiner finds that a claim recites subject matter that is not adequately supported by the original disclosure. Examiners must articulate a clear rationale for the rejection, identifying the specific claim limitations that lack support and explaining why a POSITA would not have understood the inventor to have possession of those aspects of the invention.

Applicants can overcome such rejections by amending the claims to conform to the disclosure, submitting arguments demonstrating adequate support, or providing evidence, such as affidavits or experimental data, that clarifies what was conveyed in the original specification. Ultimately, the written description requirement ensures that patent rights are granted only for what was actually disclosed and possessed by the inventor at the time of filing.

Practical Implications for Patent Drafting and Prosecution

The written description requirement under 35 U.S.C. § 112(a) has significant consequences for both the drafting of patent applications and their subsequent prosecution before the USPTO. Failure to satisfy this requirement can result in rejection of claims or invalidation during litigation. As such, applicants and practitioners must proactively draft specifications and claims that clearly demonstrate the inventor's possession of the full scope of the claimed invention as of the filing date. Below are practical guidelines and strategies to help meet this standard.

Tips for Drafting Patent Applications with Sufficient Written Description

To minimize the risk of written description rejections, patent drafters should adopt several best practices:

1. Use Consistent Terminology: Ensure that the claim language mirrors the terminology used in the specification. Variations in wording can lead to disputes over whether the specification supports a given claim term. Terminological consistency reinforces the appearance of possession and facilitates claim interpretation.
2. Support Broad Claims with Specific Examples: When drafting claims directed to broad subject matter—such as genus claims or functionally defined inventions—it is critical to include multiple specific embodiments or representative species in the specification. This is especially true in unpredictable fields such as biotechnology or chemistry.
3. Include Structural Detail: Whenever possible, describe the invention with structural features that help distinguish it from prior art or other embodiments. For chemical inventions, this may include chemical formulas, sequence listings, or three-dimensional conformations.
4. Avoid Sole Reliance on Functional Language: While functional claim language is not per se improper, relying solely on what the invention does without describing how it does it or what it consists of may result in a written description rejection. Courts often invalidate claims that attempt to capture broad functionality without providing the underlying structure or sufficient examples.
5. Fully Describe Essential or Critical Features: Pay special attention to elements that are essential to achieving the stated purpose of the invention. If a feature is critical to the function or novel aspects of the invention, it must be explicitly and thoroughly described to avoid claims being unsupported.

Overcome Written Description Rejections

When a patent examiner issues a written description rejection under 35 U.S.C. § 112(a), the applicant must demonstrate that the specification, as originally filed, sufficiently conveys to a person of ordinary skill in the art (POSITA) that the inventor was in possession of the claimed invention at the time of filing. To rebut such a rejection, applicants can rely on a variety of forms of objective evidence, so long as the evidence is consistent with and supported by the disclosure in the application as originally filed. Below are common and effective strategies for overcoming written description rejections during prosecution.

Drawings and Figures

Illustrative drawings can be powerful tools in demonstrating possession, particularly in mechanical, electrical, or design-related inventions. The Federal Circuit and USPTO have acknowledged that visual depictions of a claimed structure, when detailed and precise, can serve as adequate written description support. For example, if a claim recites a particular configuration or physical relationship between components, a properly annotated figure showing these features may establish that the inventor was in possession of the claimed subject matter.

To be persuasive, the figures must convey the same limitations recited in the claims. Mere high-level illustrations, such as conceptual block diagrams, may be insufficient unless coupled with written explanation in the specification.

Experimental Data

Experimental results and empirical data, even if not expressly included in the original application, may help show possession when they are consistent with the disclosure and aid in understanding the invention’s operation. For instance, in the chemical and biotech arts, data demonstrating binding affinity, therapeutic efficacy, or functional behavior can support broad functional claims—provided the claims do not exceed the scope of what the data substantiate.

Although post-filing data cannot substitute for original disclosure, the USPTO and courts permit the use of such data to corroborate that the original disclosure supported a claimed function or structure.

Actual Reduction to Practice

Where applicable, applicants may bolster written description by showing that the invention was actually reduced to practice before the filing date. If the inventor had physically constructed and tested the invention, and this is described in the specification or demonstrated through internal documentation consistent with the original disclosure, it can serve as compelling evidence of possession.

Reduction to practice must be clearly described or supported by corroborating documentation, such as lab notebooks, photos, or signed reports, especially in contested proceedings such as interferences or derivation proceedings.

Prosecution Strategy

If an examiner issues a written description rejection, the first step is to closely analyze the originally filed specification and claim language to determine whether there is a reasonable basis to infer possession. It may be possible to argue that the disclosure, even if not express, would convey the claimed invention to a person skilled in the art.

If not, consider narrowing the claim to match disclosed embodiments or amending the language to better align with the written description. Alternatively, where the current application cannot support the full scope of the claim, applicants may file a continuation-in-part (CIP) or rely on a parent application to establish priority and supplement the disclosure.

By understanding and anticipating written description issues, applicants can draft robust applications and implement effective prosecution strategies that minimize delays and strengthen claim validity.

Conclusion

The written description requirement remains a critical filter in the U.S. patent system, ensuring that claims define only that which was truly invented. The requirement serves not just to prevent overclaiming, but to reinforce the patent law quid pro quo—exclusive rights in exchange for public disclosure. Whether through the lens of functional language, structural chemical formulas, or descriptive means, compliance with the written description requirement must be evaluated on a case by case basis using an objective standard grounded in what a person skilled in the art would understand.

Ultimately, as courts continue determining compliance, inventors must appreciate that the written description is not merely a formality but a substantive mandate of reasonable clarity, grounded in the reality of skilled artisans and the public interest.

© 2025 Sierra IP Law, PC. The information provided herein does not constitute legal advice, but merely conveys general information that may be beneficial to the public, and should not be viewed as a substitute for legal consultation in a particular case.