Cdc34 is a ubiquitin-conjugating enzyme that participates in the ubiquitin-mediated proteolysis of a number of cell cycle progression regulatory proteins, including Sic1 (a cyclin dependent kinase inhibitor) and the G1 cyclins. A mutation in CDC34 causes cells to arrest with multiple, elongated buds and unreplicated DNA at the G1 to S phase transition. To identify genes that encode other proteins involved in this process, we constructed a library of diploid mutants that are heterozygous for random, transposon-tagged, disruption mutations and then screened for mutations that are lethal and cause a terminal morphology similar to that of a cdc34 mutant. MEB1 (multiple elongated buds) is one of the novel genes identified. Its predicted amino acid sequence of 368 residues is 20% identical to that of Cdc28, and 10 of the 11 major subdomains observed in kinase catalytic domains are evident. A disruption in this gene causes cells to die with multiple, elongated buds and only a single nucleus. We are currently investigating the possible kinase activity of the encoded protein as well as potential interactions with CDC34.