Cdc7 kinase and its activator Dbf4 protein, originally identified in budding yeast Saccharomyces cerevisiae, are widely conserved in eukaryotes including fission yeast and human. Dbf4-related activators bind and stimulate kinase activity of Cdc7-like catalytic subunit. Its kinase activity is cell cycle-regulated, mainly through availability of the activation subunit whose level increases at G1/S boundary and is maintained at a high level throughout S phase. MCM2 protein is among physiologically important substrates. Genetic studies in fission yeast indicate that Cdc7-related kinase complex also functions in meiosis, uninduced mutagenesis, DNA replication checkpoint signaling and maintenance of chromatin structures during S phase.