PTMA and xeroderma pigmentosum: The six genes showing association with the clinical severity of the XP-complementation groups involve genes in functionally relevant pathways including DNA repair (ERCC-1), immunosurveillance (interleukin 2 receptor alpha), cell cycle control (prothymosin alpha), stress response (ubiquitin C), growth factors (hepatoma derived growth factor) as well as cytoskeleton/motility proteins (tubulin alpha) as shown in Table 2.