T4 DNA Ligase can catalyze the formation of a phosphodiester bond between juxtaposed 5' phosphate and 3' hydroxyl termini in duplex DNA or RNA. This enzyme will join blunt end and cohesive end termini as well as repair single stranded nicks in duplex DNA and some DNA/RNA hybrids. T4 DNA ligase will seal nicks for these DNA substrates. Applicable to Cloning of restriction fragments, joining linkers and adapters to blunt-ended DNA.