Where a single claim defines (chemical or non-chemical) alternatives, i.e. a so-called "Markush grouping", unity of invention should be considered to be present if the alternatives are of a similar nature (see F-IV, 3.7).
When the Markush grouping is for alternatives of chemical compounds, they should be regarded as being of a similar nature where:
A "significant structural element is shared by all of the alternatives" where the compounds share a common chemical structure which occupies a large portion of their structures, or, in case the compounds have in common only a small portion of their structures, the commonly shared structure constitutes a structurally distinctive portion in view of existing prior art. The structural element may be a single component or a combination of individual components linked together. The alternatives belong to a "recognised class of chemical compounds" if there is an expectation from the knowledge in the art that members of the class will behave in the same way in the context of the claimed invention, i.e. that each member could be substituted one for the other, with the expectation that the same intended result would be achieved.
There is no need for the significant structural element to be novel in absolute terms (i.e. novel per se). Rather, this expression means that in relation to the common property or activity there must be a common part of the chemical structure which distinguishes the claimed compounds from any known compounds having the same property or activity. However, if it can be shown that at least one Markush alternative is not novel, unity of invention should be reconsidered. In particular, if the structure of at least one of the compounds covered by a Markush claim is known together with the property or technical effect under consideration, this is an indication of lack of unity of the remaining compounds (alternatives).