Eukaryotic DNA ligases are ATP-dependent enzymes that catalyse the joining of single and double-strand DNA breaks, which is an essential final step in DNA replication, recombination and repair. Four biochemically distinct DNA ligases, termed ligases I-IV, have been identified in mammalian cells. DNA ligase I is functionally homologous to the DNA ligase encoded by the Saccharomyces cerevisiae Cdc9 gene. The joining of Okazaki fragments during lagging strand DNA replication in mammalian cells is due to DNA ligase I.