BMH 71-18mutS and ES1301mutS are mismatch repair minus strains ofE. coli.Use of these strains prevents repair of the newly synthesized unmethylated strand, leading to high mutation efficiencies. Both ES1301 and BMH 71-18mutS arerecA+, therefore inserts containing highly repetitive sequences may be unstable. ES1301 is Tets, which allows for cycling between antibiotic resistances. BMH 71-18 is Tetr; therefore, it can only be used to select Ampror CATrmutants.