Claims that are broader than the supporting disclosure may be held to be invalid. Thus if the claim excludes a feature that the description indicates is essential such a claim may be invalid, particularly if the inventor admits that he only thought of omitting the feature from the claim after seeing the products of competitors. [34]

In general problems can arise as to whether this requirement was met when the applicant amends the claims during prosecution of the application. In view of the Federal Circuit’s view that the purpose of this requirement is to determine whether or not the inventor had possession of the invention at the time of filing the application, issues can arise, especially in the area of biotechnology or other areas of technology where there is a low degree of predictability, even with the original claims, notwithstanding the fact that the original claims themselves are deemed to be part of the original disclosure. [35] In such a case, in order to comply with this requirement, it is necessary that "the disclosure of the application relied upon reasonably conveys to the artisan that the inventor had possession at [the time of filing] of the later claimed subject matter". [36] In 1997, in University of California v. Eli Lilly and Co. [37] the Court of Appeals for the Federal Circuit affirmed a lower court decision that a patent which included claims directed to mammalian, vertebrate and human cDNA was invalid because there was no written description of the cDNA. The Federal Circuit noted that a written description of DNA "requires a precise definition, such as by structure, formula, chemical name or physical properties". The Court stated that describing a method for preparing cDNA or describing the protein coded for by the cDNA while describing how to make and use the cDNA did not meet the written description requirement. A prior decision in Fiers v. Sugamo [38] held that:

The moral to be drawn from these cases is that when drafting one's application one should try to ensure that basis has been provided for as many different ways of claiming the invention as possible, for example by providing bases for claiming subgeneric groupings and sub combinations of integers so that if one ever needs to fall back to claims for such limited views of the invention one can point to a specific written description of them in the original text.