The CDC40 gene product, a member of the family of proteins containing Beta-transducin-repeats, plays a major role in the regulation of the mitotic and meiotic cell cycle of the yeast Saccharomyces cerevisiae. When cells carrying the cdc40-1 t.s. mutation are grown at the restrictive temperature, DNA replication is incomplete and spindle assembly and maintenance are defective. In meiosis the CDC40 gene product is required for recombination and for progression through the first meiotic division. The CDC40 gene product is essential only at the restrictive temperature since strains bearing a CDC40 deletion allele confer a t.s. phenotype indistinguishable from the cdc40-1 cells. In order to identify genes involved in the CDC40pathway or in pathways that overlap with it, we have searched for mutations that can suppress the t.s. phenotype in strains carrying the deletion allele. All the mutants isolated fell into one complementation group. We call this gene SCF1 (Suppressor of Cdc Forty). The gene was located to chromosome VIII. We are now pursuing its characterization. In a different approach, we have used a color colony synthetic lethal screen in an attempt to identify genes that may bypass the requirement for CDC40 at the permissive temperature. Out of 25,000 mutagenized colonies 50 synthetic lethal mutants were isolated, which could not survive at the permissive temperature without the presence of CDC40 on a plasmid. Complementation analysis and cloning of the genes are now under way.