XPC and xeroderma pigmentosum: In our previous report, we failed to identify etiologic mutations in the XPC gene in XP40OS cells which had been classified as the complementation group C. Since xeroderma pigmentosum is an autosomal recessive disorder and its frequency in the Japanese population is one in 22,000 [2], we searched for rare variants or mutations resulting in a homozygous or compound heterozygous state.