M13mp18 is the double-stranded, covalently closed, circular form of DNA derived from bacteriophage M13. This phage vector contains single HindIII, SphI, SbfI, PstI, SalI (AccI/ HincII), XbaI, BamHI, SmaI (XmaI), KpnI (Acc65I), SacI and EcoRI sites within the β-Galactosidase gene (1). When a fragment of DNA is inserted into one of these sites, the β-Galactosidase gene is inactivated, providing selection for clones on the appropriate indicator plate (2).