This question was answered in a very recent Federal Circuit case, Inphi Corp v. Netlist Inc., an appeal from the Patent Trial and Appeal Board’s (PTAB) final written decision in an Inter Partes Review (IPR). The petitioner challenging validity argued that the specification did not provide enablement under section 112, first paragraph, for the exclusion of the elements, so the exclusion was new matter.

Previous law had been clear that a claim can contain a negative list excluding elements in a class if the specification provides differences between those excluded and those retained. Santarus, Inc. v. Par Pharm., Inc., 694 F.3d 1344, 1351 (Fed. Cir. 2012).

“[n]egative claim limitations are adequately supported when the specification describes a reason to exclude the relevant limitation”

The Netlist patent’s specification had included groupings of the excluded elements without the express advantages and disadvantages of each. The party challenging the patent claimed that the “reason to exclude” must be explained in so many words in the specification. The Federal Circuit declined to extend the law that far, holding that Santarus “did not create a heightened written description standard for negative claim limitations and that properly described, alternative features are sufficient to satisfy the written description standard of § 112, paragraph 1, for negative claim limitations.” The new rule is that the “reason to exclude” need not be expressly set out but could be implicit in the groupings of the elements.

The takeaway message for prosecution then is that, if an applied-for invention contains many elements in a larger class, the specification should group the elements into subgroups and explain why the elements may be grouped separately. It is impossible to know exactly which references will surface during prosecution, and grouping elements of a class into subgroups will at least provide opportunity to get claims issued for the subgroup. If there are inherent differences in elements, the prosecutor should say what she can about those differences. Section 112, first paragraph (now 112(a)), can be satisfied if groupings of possible elements are differentiated by implication.