Inhibitors of mismatch repair can be used to generate hypermutable cells and organisms. By inhibiting this process in cells, new cell lines and varieties with novel and useful properties can be prepared more efficiently than by relying on the natural rate of homologous recombination. These methods are useful for generating targeted loci that can alter the expression profiles of target genes as well as tag exons of a gene with a reporter marker to facilitate the monitoring of a given gene product when the host is grown under different conditions or exposed to biological and chemical entities.