The frequencies of mutant erythrocytes with loss of heterozygosity at the glycophorin A (GPA) locus and mutant CD4+ T cells lacking surface expression of the T-cell receptor alphabeta (TCR)/CD3 complex were measured by flow cytometry for Japanese Werner's syndrome (WRN) patients. The hemizygous and homozygous GPA mutant frequencies (GPA Mfs) and the TCR/CD3-defective mutant frequency (TCR Mf) in WRN patients were found to be significantly higher than those in normal controls in the same age range. However, because these Mfs in the patients are only about twice those in controls, it is difficult to conclude that the WRN gene mutations cause instability of somatic genes. This contrasts markedly with Bloom's syndrome (BLM) patients, whose GPA and TCR Mfs were previously reported to increase about 50- and 15-fold, respectively. The difference in Mfs is one aspect of the large variation in the phenotype observed between WRN and BLM patients, suggesting a different role of the responsible genes, both of which belong to the RecQ DNA helicase gene family, in the control of somatic mutagenesis.