Abstract: An assay is disclosed for identifying mutagenic agents which include large, multilocus deletion events in cultured mammalian cells. The cell lines used are adenine phosphoribosyltransferase heterozygotes derived from CHO cells, which we disclose possess a genomic sequence variation near the above-mentioned gene. We present the nucleotide sequence surrounding this variation, enabling an efficient determination of the proportion of deletion mutants among the mutant colonies induced by the agent to be tested.