Can a Markush group in a patent claim be indefinite?

This page is an FAQ based on guidance from the Manual of Patent Examining Procedure. It is provided as guidance, with links to the ground truth sources. This is information only: it is not legal advice.

Yes, a Markush group in a patent claim can be indefinite under certain circumstances. The MPEP 2173.05(h) states:

“In certain circumstances, however, a Markush group may be so expansive that persons skilled in the art cannot determine the metes and bounds of the claimed invention. For example, if a claim defines a chemical compound using one or more Markush groups, and that claim encompasses a massive number of distinct alternative members, the claim may be indefinite under 35 U.S.C. 112(b) if one skilled in the art cannot determine its metes and bounds due to an inability to envision all of the compounds defined by the Markush group(s).”

In such cases, a rejection for indefiniteness under 35 U.S.C. 112(b) may be appropriate. However, breadth alone does not necessarily make a Markush group indefinite.