To understand the molecular mechanisms of XP, XP mouse models have been used, and mice deficient in XPA, XPC, XPD, XPG, XPF, and XPA/CSB have been produced and analysed. A recent elegant technique of targeting gene replacement in mouse embryonic stem cells has provided researchers with the ability to generate mutant mice defective in any specific gene(s). 32 Animals generated in this way display phenotypes and symptoms of XP patients, and have provided valuable tools to understand how and where the deficiency in DNA repair may lead to tumor formation, and also in studies of developmental biology and the aging process. Mouse studies have recently contributed to our understanding of the role of ink4a-Arf in increasing the risk of melanoma photocarcinogenesis in an XPC mutant background. As with many other genetic defects, the distribution of XP globally is not uniform. In most cases the frequency of mutation of a particular trait depends when and where a specific mutation arose.