A more thorough explanation:

The Markush doctrine is an exception to the general rule against using alternative language in patent claims. It is commonly used in claims that involve chemical components. The doctrine allows a claimant to use an alternative, subgeneric phrase when there is no commonly accepted generic expression available.

For example, a claim may include a phrase such as "selected from the group consisting of," which means that the claim covers a group of substances that could all serve the same function in a process. This allows the claimant to cover a range of possible alternatives without having to list them all individually.

The Markush doctrine was named after Dr. Eugene A. Markush, who was granted a dye-preparation patent in 1923. The doctrine was established in the case of Ex parte Markush in 1925.

Here is an example of how the Markush doctrine might be used in a patent claim:

"A composition comprising a material selected from the group consisting of X, Y, and Z, wherein the material is effective in treating a particular medical condition."

This claim covers a range of possible materials that could be used to treat the medical condition, without having to list them all individually. This makes the claim more flexible and easier to work with.