Comparative Medicine

µm), has been shown to be the most important factor leading to nonmelanoma skin cancer.24 Epidemiologic studies revealed that the incidence of skin cancer has been steadily increasing recently, and approximately 1 million new cases of nonmelanoma skin cancer occur in the United States annually.17 Acute and chronic exposure to UV radiation also increases the risk of basal cell car-cinoma and malignant melanoma.19 This association has been demonstrated in the hairless mouse model of skin carcinogenesis, in which development of skin tumors is dependent on the dose and duration of UV radiation, especially UVB.8 The skin carcinogenesis induced by UVB starts with the initia-tion of DNA damage15 due to the formation of pyrimidine (6 to 4 photoproducts) and cyclobutane pyrimidine dimers, thymine dimers, and reactive oxygen species.11,23 Exposure of mammalian cells to UVB leads to activation of genes involved in cell prolifera