Two families of cyclin-like proteins have been found in S. cerevisiae. The clb proteins are the mitotic cyclins. The cln proteins provide an essential function, are required for the G1/S transition, and appear to be rate-limiting for START, but have no obvious role elsewhere in the cycle. The cln proteins are unstable; they form complexes with cdc28; the complexes have protein kinase activity; and at least one of the clns oscillates in abundance through the cell cycle. The action of the cln cyclins at START suggests that they may be 'G1 cyclins'.