The cell division cycle gene, CDC34, is required for ubiquitin-mediated degradation of G(1) regulators and cell cycle progression through the transition from G(1) to S phase in budding yeast. A CDC34 requirement for S phase onset in higher eukaryotes has not been established, Studies of the simple embryonic cell cycle of Xenopus laevis eggs demonstrated that Cdc34p in a large molecular size complex was required in the initiation of DNA replication, Cdc34p appears to regulate the initiation function of Cdk2-cyclin E, perhaps through the degradation of the Xenopus cdk inhibitor, Xic1.