We have cloned CYR1, the S. cerevisiae gene encoding adenylate cyclase. The DNA sequence of CYR1 can encode a protein of 2026 amino acids. This protein would contain a central region comprised of over twenty copies of a 23 amino acid repeating unit with remarkable homology to a 24 amino acid tandem repeating unit of a trace human serum glycoprotein. Gene disruption and biochemical experiments indicate that the catalytic domain of adenylate cyclase resides in the carboxyl terminal 400 amino acids. Elevated expression of adenylate cyclase suppresses the lethality that otherwise results from loss of RAS gene function in yeast. Yeast adenylate cyclase, made in E. coli, cannot be activated by added RAS protein.