To answer this, we compared the biochemical properties of XPF-ERCC1 from two patients, XP42RO (a patient with mild XP, homozygous for a mutation causing an R799W substitution in XPF[33]) and XP51RO (a patient with XFE progeroid syndrome, homozygous for a mutation causing an R153P substitution in XPF[32]) to that of wild type XPF-ERCC1. The gene discussed is ERCC4; the disease is xeroderma pigmentosum.