While many patent examiners prefer Markush claims, which recite lists of alternatively useable species, to be formatted as “selected from the group consisting of A, B, and C,” Applicants should carefully consider claim construction before adopting this format. “A trap for the unwary claim drafter using Markush group language is the severely closed nature of a Markush group.” 3-8 Chisum on Patents § 8.06 (2017).

Unless other language alters the meaning of a Markush clause, courts construe Markush groups as closed. See Multilayer Stretch Cling Film Holdings, Inc. v. Berry Plastics Corp., 831 F.3d 1350, 1358 (Fed. Cir. 2016) (“The presumption that a claim term set off by the transitional phrase ‘consisting of’ is closed to unrecited elements is at least a century old and has been reaffirmed many times by our court and other courts.”). “[T]o overcome the exceptionally strong presumption that a claim term set off with ‘consisting of’ is closed to unrecited elements, the specification and prosecution history must unmistakably manifest an alternative meaning.” Id. at 1359 (emphasis added). In Multilayer, the Federal Circuit examined the scope of the transitional phrase “consisting of” and held that a Markush group in which a list of resins was preceded by “consisting of” excluded unlisted resins, even though the specification recited other suitable resins and a dependent claim recited a resin not listed in its corresponding independent claim. Id. at 1360-61. However, intrinsic evidence demonstrated that the Markush group was open to blends because one listed resin was a sub-type of another listed resin, overcoming the “presumption that a Markush group is closed to mixtures of the listed elements.” Id. at 1363-64.

Similarly, the Federal Circuit reversed an infringement decision in Shire Dev., LLC v. Watson Pharms., Inc., 848 F.3d 981 (Fed. Cir. 2017) because the Markush group requirements were not satisfied. The alleged infringing product included “magnesium stearate [which] structurally and functionally relate[d] to the invention,” a controlled release oral formulation. Id. at 986. Accordingly, even though Shire argued that examples in the specification disclosed magnesium stearate in the outer matrix, its presence violated the claimed “consisting of” Markush limitation for the outer hydrophilic matrix. Id.

To avoid unnecessarily restrictive claim constructions, Applicants who intend Markush groups to include mixtures of listed elements or unrecited elements should carefully consider every clause in the grouping. For example, using transitional clauses without closed language such as “selected from A, B, and C” or “is A, B, or C” might bypass the construction issues faced by patent owners in Multilayer and Shire, cases in which the Federal Circuit focused on the closed nature of “consisting of.” Additionally, where the use of closed language avoids prior art, “selected from the group consisting essentially of” might allow for more latitude in claim construction than its “consisting of” counterpart. Moreover, to cover mixtures, use of preambles like “at least one X” or “one or more X” or clauses such as “and mixtures thereof” expressly open Markush groups to combinations and blends. While the case law on Markush groups is evolving, conscientious drafting may enable Applicants to sidestep known traps.

An additional discussion of the Multilayer decision and Markush groups can be in Finnegan’s Full Disclosure newsletter here.