M13KO7 is an M13 derivative, with the origin of replication from P15A and the kanamycin resistance gene from Tn903 both inserted within the M13 origin of replication (1). M13KO7 is able to replicate in the absence of phagemid DNA. In the presence of a phagemid bearing a wild-type M13 or f1 origin, single-stranded phagemid is packaged preferentially and secreted into the culture medium.This allows easy production of single-stranded phagemid DNA for mutagenesis or sequencing.