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A "Markush expression," also known as a "Markush group" or "Markush structure," is a type of patent claim following the form "An X selected from the group consisting of Y1, Y2, and Y3." The form is most common in chemical patents, but pops up in other contexts as well. Probably the most well-known Markush expression comes from a dependent claim of U.S. Patent No. 3,234,948:

2. A cigarette filter according to claim 1, in which the cheese comprises grated particles of cheese selected from the group consisting of Parmesan, Romano, Swiss and cheddar cheeses.

Markush expressions can run much longer than this: for an example, see the chemical patent at this link.

In general, the elements of a Markush expression have to belong to some sort of class recognized in the art. You could, for instance, cite a group consisting of helium, neon, argon, krypton, xenon and radon, since any chemist would recognize this group as the noble gases. You could cite a group consisting of gasoline, avgas and jet fuel, since they form a coherent group of vehicular fuels. If the members of the group have no discernible elements in common, the claim is probably invalid.

Assuming the Markush expression is valid on this point, each item in the claim must be examined separately for obviousness, prior art, and other issues that can thwart a patent claim. If any of the items fails, the entire claim is rejected.

The term comes from Eugene Markush, an inventor who was the first person to get a patent using such a claim structure.

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