Results of PCR assays for mutations at position G238C (A), G460A (B), and A719G (C). Photographs of ethidium bromide-stained gels are shown. “M” represents marker lanes. In (A) “A” indicates the result with wild-type and “B”, the result with mutation-specific primers. In (B and C), “−” indicates the result before digestion and “+” after digestion with restriction enzymes, as descibed in Materials and Methods. Homozygous mutated DNA was obtained from cases with absent TPMT activity not included in the present study.