Three‐allele restriction fragment length polymorphisms (RFLPs) for the restriction endonuclease Bam HI are known at the hypoxanthine‐guanine phosphoribosyltransferase (HPRT, E.C.2.4.2.8.) gene locus. The alleles are expressed phenotypically on Southern blots as three distinct pairs of fragments that hybridize to HPRT cDNA: i)a 22‐kilobase (kb) / 25‐kb pair, ii) a 12‐kb / 25‐kb pair, and iii) a 22‐kb / 18‐kb pair. Allele frequencies in 119 unrelated Japanese people were 0.38 for the 22‐kb / 25‐kb allele, 0.43 for the 12‐kb / 25‐kb allele, and 0.19 for the 22‐kb / 18‐kb allele, an average heterozygosity of 66% in Japanese females, a higher rate than in Caucasian females. Five out of nine carriers of partial or complete HPRT deficiency showed heterozygou