A more thorough explanation:

A Markush group is a limited form of generic claim used in patents. It involves reciting an element, stating that the element is a member of a group, and naming the other group members, any of which could substitute for the first recited element. All Markush group members must have at least one common property that is mainly responsible for their function in the claimed relationship. Although each material in the group is different, each must be able to serve the same function.

A patent claim for a sunscreen lotion may use a Markush group to describe the UV-absorbing agents used in the lotion. The claim may state that the lotion contains a UV-absorbing agent selected from the group consisting of benzophenone-3, octinoxate, and avobenzone. Any of these agents could be used in the lotion, as long as they serve the same function of absorbing UV rays.

The Markush doctrine is an exception to the policy against the use of alternative language in patent claims. It is used in certain claims, especially those involving chemical components, when there is no applicable, commonly accepted generic expression. The claimant can use an alternative, subgeneric phrase that includes a group of substances any one of which could serve the same function in the process. The term Markush comes from Dr. Eugene A. Markush, who was granted a dye-preparation patent in 1923.

A patent claim for a pharmaceutical composition may use a Markush group to describe the active ingredients used in the composition. The claim may state that the composition comprises an active ingredient selected from the group consisting of compound A, compound B, and compound C. Any of these compounds could be used in the composition, as long as they serve the same function of treating the targeted disease.