Publisher Summary This chapter presents specific protocols for determining a suitable selecting concentration of drug to use in the isolation of single-step mutants. When cells are gradually adapted to increasing concentrations of a drug, they frequently respond by amplification of the gene encoding the target protein for that drug. The classical example of this phenomenon is the amplification of the dihydrofolate reductase gene seen in the presence of methotrexate. Chinese hamster ovary (CHO) cells have been used as a model system for the isolation of drug-resistant mutants. There are many advantages to the isolation of drug-resistant mutants. As the drug targets the mutation to a specific protein or metabolic process, initial biochemical characterization of the mutant is much easier. Drug-resistant mutants frequently have other properties which allow the investigator to begin the genetic analysis of a system which otherwise would be very complicated. Another kind of conditional drug-resistant mutant may fail to grow in the absence of the drug (drug dependence). In this instance, selection of drug-independent revertants can be used in the same way as selection of temperature-resistant revertants.