BMH 71-18 mutS and ES1301 mutS are mismatch repair minus strains of E. coli. Use of these strains prevents repair of the newly synthesized unmethylated strand, leading to high mutation efficiencies. Both ES1301 and BMH 71-18 mutS are recA+, therefore inserts containing highly repetitive sequences may be unstable. ES1301 is Tet(s), which allows for cycling between antibiotic resistances. BMH 71-18 is Tet(r); therefore, it can only be used to select Amp(r) or CAT(r) mutants. ES1301 mutS Genotype: lacZ53, mutS201::Tn5, thyA36, rha-5, metB1, deoC, IN(rrnD-rrnE). BMH 71-18 mutS Genotype: thi, supE, Delta(lac-proAB), [mutS::Tn10], [F', proAB, lacI(q)ZDeltaM15].|