We have isolated a number of cDNAs derived from human mRNAs that are able to substitute for G1 cyclin genes in S. cerevisiae. Several of these encode human cyclins A, B1, and B2. Three novel genes have been identified, which we call cyclins C, D, and E. The novel proteins are sufficiently distantly related to the other members of the cyclin family and to each other as to constitute three new classes of cyclins. Cyclin C and E mRNAs accumulate periodically through the cell cycle, peaking at different times in G1.