In Chinese hamster ovary (CHO) cells, heterozygotes for the adenine phosphoribosyltransferase (APRT) locus arise spontaneously at high frequencies. Paradoxically, such heterozygotes yield APRT mutants only at much lower spontaneous rates, suggesting that the high-frequency event may occur at only one of the two APRT genes. In an attempt to understand the genetic basis for the apparent refractivity of one of the APRT alleles to the high-frequency genetic event and to determine whether differences in the genomic environments of the two CHO APRT alleles specifically render one gene more susceptible to high-frequency spontaneous deletion or inactivation, we have mapped the wild-type APRT allele in 16 independently derived spontaneous APRT heterozygotes. In 15 of these 16 heterozygotes, the functional, wild-type APRT gene was found to reside on the Z7 chromosome, indicating that the high-frequency event is indeed highly specific for the Z4 APRT allele. All but one of these heterozygotes were hemizygous for the APRT locus, suggesting that the high-frequency event generally involves deletion rather than spontaneous inactivation or mutation of the Z4 APRT allele.