The c-Jun N-terminal kinase (JNK) signaling pathway is known to mediate both survival and apoptosis of tumor cells. Although JNK1 and JNK2 have been shown to differentially regulate the development of skin cancer, the underlying mechanistic basis remains unclear. Here, we demonstrate that JNK1, but not JNK2, interacts with and phosphorylates Mytl ex vivo and in vitro. UVA induces substantial apoptosis in JNK wild-type (JNK +/+) or/Mo-deficient (JNK2 -/-) mouse embryonic fibroblasts but has no effect on JNKl-defkient (JNK1 -/-) cells. In addition, UVA-induced caspase-3 cleavage and DNA fragmentation were suppressed by the knockdown of human Mytl in skin cancer cells. JNK1 deficiency results in suppressed Mytl phosphorylation and caspase-3 cleavage in skin exposed to UVA irradiation. In contrast, the absence of JNK2 induces Mytl phosphorylation and caspase-3 cleavage in skin exposed to UVA. The overexpression of JNK1 with Mytl promotes cellular apoptosis during the early embryonic development of Xenopus laevis, whereas the presence of JNK2 reduces the phenotype of Mytl-induced apoptotic cell death. Most importantly, JNK1 -/- mice developed more UVA-induced papillomas than either JNK +/+ or JNK2 -/- mice, which was associated with suppressed Mytl phosphorylation and decreased caspase-3 cleavage. Taken together, these data provide mechanistic insights into the distinct roles of the different JNK isoforms, specifically suggesting that the JNKl-mediated phosphorylation of Mytl plays an important role in UVA-induced apoptosis and the prevention of skin carcinogenesis.