E. coli BMH 71-18 mutS, which is mismatch-repair deficient, is used to propagate mutated plasmids. Two rounds of DNA digestion and transformation ensure that a very high frequency of transformants carry the mutated plasmid, which nearly always contains both mutations—the desired mutation and the selection mutation (Deng and Nickoloff 1992; Zhu 1996). Not available in all regions.