Restriction Fragment Length Polymorphism (RFLP) related to cystatin gene (CST) family was detected in the Japanese population by using restriction enzyme Sac I. A polymorphic site, located at 0.9 kb from the 3' end of the CST2 gene, revealed a two allele polymorphism with band sizes of 3.5 kb and 8.3 kb by hybridization with probe including exon 2 of the CST1 gene. The gene frequencies in the Japanese population were 0.326 for 3.5 kb allele and 0.674 for 8.3 kb allele (n = 86). The phenotypes of the polymorphism showed no association with the previously reported electrophoretic cystatin SA protein phenotypes.