DNA ligase is a specific type of enzyme, a ligase that facilitates the joining of DNA strands together by catalyzing the formation of a phosphodiester bond. DNA ligase is used in both DNA repair and DNA replication. In mammals, there are four specific types of ligase:DNA ligase I (ligates the nascent DNA of the lagging strand after the Ribonuclease H has removed the RNA primer from the Okazaki fragments.); DNA ligase III (complexes with DNA repair protein XRCC1 to aid in sealing DNA during the process of nucleotide excision repair and recombinant fragments. Of the all known mammalian DNA ligases, only Lig III has been found to be present in mitochondria.); DNA ligase IV (complexes with XRCC4. It catalyzes the final step in the non-homologous end joining DNA double-strand break repair pathway. It is also required for V(D)J recombination, the process that generates diversity in immunoglobulin and T-cell receptor loci during immune system development.); DNA ligase II (appears to be used in repair. It is formed by alternative splicing of a proteolytic fragment of DNA ligase III and does not have its own gene, therefore it is often considered to be virtually identical to DNA ligase III.).