Isoform 3 functions as heterodimer with DNA-repair protein XRCC1 in the nucleus and can correct defective DNA strand-break repair and sister chromatid exchange following treatment with ionizing radiation and alkylating agents. Isoform 1 is targeted to mitochondria, where it functions as DNA ligase in mitochondrial base-excision DNA repair (PubMed:10207110, PubMed:24674627). ATP + (deoxyribonucleotide)n-3'-hydroxyl + 5'-phospho-(deoxyribonucleotide)m = (deoxyribonucleotide)n+m + AMP + diphosphate. Isoform 3 interacts (via BRCT domain) with the nuclear DNA-repair protein XRCC1. Contains an N-terminal mitochondrial transit peptide. Contains an N-terminal mitochondrial transit peptide. Lacks the N-terminal mitochondrial transit peptide. Lacks the N-terminal mitochondrial transit peptide. Testis, thymus, prostate and heart. The PARP-type zinc finger is required for DNA ligase activity. Produced by alternative splicing. Produced by alternative splicing. Produced by alternative initiation of isoform 1. Produced by alternative initiation of isoform 2. Belongs to the ATP-dependent DNA ligase family. DNA ligase entry