We have cloned the CDC25 gene of Saccharomyces cerevisiae whose product is required for traversing the Go phase of the cell cycle. A preliminary physical characterization of the CDC25 gene region is presented. In addition, we show that another gene, when cloned in a high-copy number plasmid, is able to partially suppress growth thermosensitivity of a strain carrying the cdc25 mutation. We briefly discuss the possible interaction of these gene products with adenylate cyclase encoded by the CDC35 gene.