We used gene transfer to identify frequent genetic rearrangements responsible for activating mutant genes in mammalian cells. We transformed an aprt- tk- cell with a plasmid containing a wild-type aprt gene and a truncated, promoterless tk gene. Transformants that integrate a single copy of this plasmid exhibit the aprt+ phenotype but remain tk-. Tk+ variants result from 20 to 50 fold amplification of the linked plasmid along with significant lengths of flanking DNA. They produce aberrant transcripts such that multiple genes are required to generate sufficient enzyme to convert the cell to the tk+ phenotype. One striking feature of the amplified aprt+ tk+ clones is the frequency (10(-4) ) at which aprt- tk+ mutants appear. These phenotypic requirements are such that the tk gene must remain amplified while all the amplified aprt genes become inactivated. The structure of the amplified DNA indicates that within aprt- cells, all amplified units bear identical mutations. These data suggest that these cells possess an efficient correction mechanism that maintains sequence homogeneity among repeated genetic elements.