Xeroderma pigmentosum (XP) complementation group F was first reported in Japan and most XP-F patients reported to date are Japanese. The clinical features of XP-F patients are rather mild, including late onset of skin cancer. Recently a cDNA that corrects the repair deficiency of cultured XP-F cells was isolated. The XPF protein forms a tight complex with ERCC1 and this complex functions as a structure-specific endonuclease responsible for the 5' incision during DNA excision repair. Here we have identified XPF mRNA mutations and examined levels of the mRNA and protein expression in seven primary cell strains from Japanese XP-F patients. The XP-F cell strains were classified into three types in terms of the effect of the mutation on the predicted protein; (i) XPF proteins with amino acid substitutions; (ii) amino acid substituted and truncated XPF proteins; and (iii) truncated XPF protein only. A normal level of expression of XPF mRNA was observed in XP-F cells but XPF protein was extremely low. These results indicate that the detected mutations lead to unstable XPF protein, resulting in a decrease in formation of the ERCC1-XPF endonuclease complex. Slow excision repair of UV-induced DNA damage due to low residual endonuclease activity provides a plausible explanation for the typical mild phenotype of XP-F patients.