Xeroderma pigmentosum (XP) is a human repair-deficient disorder that is caused by mutations in any of eight genes (A-G, V). The genes for complementation groups A-G have been cloned fully or in part, but the gene for the XP variant (XPV) has yet to be cloned. The lack of progress with XPV is in large part due to the rarity of stably transformed cell lines. We have attempted to immortalize fibroblasts from several XPV patients to obtain cell lines with which to characterize this disease and clone the appropriate gene. We have found, as have other investigators, that this XP group is very difficult to immortalize. We used a variety of approaches, including transfection with pSV ori- (a plasmid containing the simian virus (SV) 40 large T antigen) followed by spontaneous transformation, which provided stable immortal lines from Cockayne syndrome A and B, but not from XPV; transfection with pSV ori- and exposure to 3 Gy of X-rays; transfection with pSV ori-, exposure to 2 Gy of X-rays, and treatment with 1 mM ethyl methanesulfonate; transfection with human papilloma virus-16; and infection with SV40. Even though we used as many as 2 x 10(8) cells in some experiments, we were able to immortalize only one of our lines, XP30RO. Because the biochemical defect in XPV cell lines involves the capacity to replicate damaged DNA templates, perhaps the XPV gene product could be a replication factor that interacts with SV40 T antigen, and whose absence from XPV cell lines presents difficulties for the immortalization process to proceed.