We have isolated two Drosophila cDNA clones that rescue Saccharomyces cerevisiae deficient in CLN functions. One of these clones is the Drosophila homolog of the cdc2 gene. The second encodes a distant and new member of the cyclin family of proteins, cyclin C. It is highly homologous (72% identity) to a human clone isolated in a similar screen. Yeast cells rescued by a plasmid constitutively expressing this Drosophila cyclin C are unusually small, consistent with an unregulated high level of G1 cyclin function. Sequence comparisons identified regions conserved among the more distantly related cyclins. Based on these conserved elements, we identified homology between cyclins and the ras oncogene.