Cyclins, regulatory subunits, which associate with kinases, control many of the important steps in cell cycle progression. The Cdc2 protein kinase (p34Cdc2) exhibits protein kinase activity in vitro and exists in a complex with both cyclin B and a protein homologous to p13SUC1. Cdc2 kinase is the active subunit of the M phase promoting factor (MPF) and the M phase-specific Histone H1 kinase. The p34Cdc2/cyclin B complex is required for the G2 to M transition. An additional cell cycle-dependent protein kinase, termed p55cdc, exhibits a high degree of homology with the S. cerevisiae proteins Cdc20 and Cdc4. The p55cdc transcript is readily detectable in a variety of cultured cell lines in growth phase, but disappears when cell growth is chemically arrested.