Transient transfectron of mammalian cell lines with recombinant DNA has become a common tool for studying functional and structural properties of a wide variety of proteins (1–5). Transient transfections are especially practical for the expression of recombinant multisubunit complexes because their expression needs the prior knowledge and selection of which of a set of subunits are necessary to form functional heteromultrmers with distinct features. Subumt stoichiometry, mode of assembly, and the relative expression levels of individual cDNA vector constructs also have to be taken into account. The latter still poses an insurmountable problem, because no accurate predictions can be made.