Chromosomal variation and the origin of drug-resistant mutants in mammalian cell lines.

Drug-resistant mutants of somatic cell lines fall into two classes: one seems to correspond to classical gene mutation, the other is characterized, albeit transiently, by karyotypic instability, high reversion frequency, and low plating efficiency. An interpretation of the origin of these drug-resistant mutants is offered on the basis of chromosomal variation, which generates variation of the number of copies of each individual chromosome and, consequently, of gene dosage.