Genetic studies in S. cerevisiae indicate that wild-type Cdc6 function is required for correctly timed loading of Mcm2-7 onto ORC. Biochemical studies indicate that the human and Xenopus Cdc6 proteins likewise are required for Mcm2-7 loading, and that they are ATPase switches. Specifically, Cdc6 may function as a clamp loader, assembling Mcm2-7 onto DNA in an ATP-dependent reaction. All known Cdc6 proteins have the Walker A and Walker B sequence motifs characteristic of the AAA+ superfamily of ATPases. As expected for an AAA+ protein, human Cdc6 binds and slowly hydrolyzes ATP in vitro. ATP hydrolysis was disrupted by mutations of the Walker B motif, while both binding and hydrolysis were disrupted by Walker A mutations. Microinjection of either mutant protein into HeLa cells blocked their progression through S phase. Both wild-type and mutant proteins can dimerize in vitro, and studies with Xenopus egg extracts suggest that Cdc6 functions in vivo as a dimer or larger multimer. In Xenopus extracts depleted of Cdc6 and reconstituted with either mutant protein, recruitment of Mcm2-7 to chromatin failed.