T4 DNA Ligase is an ultrapure recombinant enzyme purified from Escherichia coli supplied with an optimized 4× Reaction Buffer, which includes ATP. T4 DNA Ligase catalyses the formation of a phosphodiester bond between juxtaposed 5′-phosphoryl and 3′-hydroxyl termini in duplex DNA. It repairs single-strand nicks in duplex DNA and will join both blunt and cohesive-end restriction fragments of duplex DNA or RNA. The enzyme requires ATP as cofactor.