Bacteriophage restriction enzyme. Molecular model of the restriction enzyme endonuclease V (yellow) from the bacteriophage T4 complexed with DNA (deoxyribonucleic acid, red and blue). Restriction enzymes, also known as restriction endonucleases, recognise specific nucleotide sequences and cut the DNA at these sites. T4 endonuclease V initiates the repair of ultraviolet light-induced damage to DNA.