The events of the eukaryotic cell cycle are governed by cyclin-dependent kinases (cdk's), whose activation requires association with cyclin regulatory subunits expressed at specific cell cycle stages. In the budding yeast Saccharomyces cerevisiae, the cell cycle is thought to be controlled by a single cdk, CDC28. Passage through the G1 phase of the cell cycle is regulated by complexes of CDC28 and G1 cyclins (CLN1, CLN2, and CLN3). A putative G1 cyclin, HCS26, has recently been identified. In a/alpha diploid cells lacking CLN1 and CLN2, HCS26 is required for passage through G1. HCS26 does not associate with CDC28, but instead associates with PHO85, a closely related protein kinase. Thus, budding yeast, like higher eukaryotes, use multiple cdk's in the regulation of cell cycle progression.