Markush groups are proper if the group members have a common functionality, or common features/structure to achieve a function. A Markush group is improper if it identifies multiple distinct inventions; any of a Markush group should be interchangeable, and thus patentably indistinct, to another in the same group.

The patent office on the situation -

While the origins of the Markush claim drafting technique lie in the chemical arts, claims that recite alternatives are now commonplace in all areas of technology. Applicants sometimes use Markush or other alternative formats to claim multiple inventions and/or to recite hundreds, if not thousands, of alternative embodiments of a single invention in one claim. Proper search of such complex claims, particularly those using Markush language, often consume a disproportionate amount of Office resources as compared to other types of claims. The prosecution of these complex claims likewise often requires separate examination and patentability determinations for each of the alternatives within the claim, e.g., if the alternatives raise separate prior art, enablement, or utility issues.

The struggle to balance the needs of inventors for coverage of the full scope of their inventions with those of the Office for search and examination responsibilities commensurate in scope with resources is a long-standing one.

Current Office policy requires examination of all species of a claim that recites a Markush group when the alternatives are sufficiently few in number or so closely related that search and examination can be made without serious burden. See MPEP 803.02 (8th Ed., Rev. 5, Aug. 2006). Consistent with the Harnisch decision, the Office cannot refuse to examine what applicants regard as their invention unless the subject matter in a claim lacks unity of invention. As a result, even where the search and examination of a claim that has ‘‘unity of invention’’ would require serious burden, the examiner must determine the patentability of the claim.

[T]o date, the Office has not established official procedures for examiners to follow when examining a claim that recites alternatives wherein the alternatives lack ‘‘unity of invention’’ or for restricting an application to one invention where multiple independent and distinct inventions are recited as alternatives in a single claim. The Office is proposing to revise the rules of practice to provide such procedures.

Besides constraining Markush group inclusion, and limiting a claim to a single invention, the proposed rules intend to simplify examination via claim construction by limiting incorporation by reference within a claim "unless there is no other practical way to define the invention," setting a required format for Markush claims that read on multiple species, and other odious claim drafting practices, in an effort to proscribe making "the claim difficult to construe."

The agency take -

To enable the Office to do a better, more thorough and reliable examination of such claims, the Office is proposing rules to limit each claim to a single invention and to define acceptable formats for claims that set forth alternatives. These proposed rules are not intended to change current restriction practice with regard to multiple independent and distinct inventions claimed in separate claims of an application. Rather, they are intended to provide a mechanism by which the Office can require a single claim to be limited to a single invention.

The Office solicits comments and suggestions, but unless you have solid input to make an examiner's life easier, you're likely tilting at a windmill. The agency has been feeling its oats lately, and is on a roll steamrolling rule changes.

For those interested in the details: today's Federal Register on the proposed rule changes.

A bit of history on resistence to USPTO Markush rule making from Hal Wegner:

An effort at a clampdown on Markush practice in the 1970's was dealt with by the bar associations in test cases, In re Weber, 580 F.2d 455 (CCPA 1978); In re Haas, 580 F.2d 461 (CCPA 1978). See also Ex Parte Hozumi,1984 Westlaw 62977, 3 U.S.P.Q.2d 1059 (PTO Bd.Pat.App & Interf. 1984).

The Weber and Haas cases are noted in the Federal Register posting by the PTO, as well as the succeeding Harnisch case 631 F.2d 716, 206 USPQ 300 (CCPA 1980), but not Hozumi.

Dennis Crouch on the proposed Markush rule changes.