An enzyme that catalyses a reaction that links two DNA molecules via the formation of a phospho-diester bond between the 3´ hydroxyl and 5´ phosphate of adjacent nucleotides. It plays an important role in DNA repair and replication. DNA ligase is one of the essential tools of recombinant DNA technology, enabling (among other things) the incorporation of foreign DNA into vectors. The ligase enzyme encoded by phage T4 is commonly used in gene-cloning experiments. It requires ATP as a co-factor. T4 is used in vitro to join the vector and insert DNAs.