Mouse Kit L575P, the ortholog of human KIT L576P, a common KIT mutation found in human melanoma was expressed in an immortalized but non-transformed mouse Ink4a-Arf-deficient melanocyte cell line. The resultant Ink4a-Arf-deficient Kit L575P-expressing melanocytes exhibited increased proliferation, the ability to grow in soft agar, and increased migration. When these cells were injected subcutaneously into NOD/SCID/gamma(c) mice, melanomas arose in 5 of 7 (71%) mice. One of seven mice (14%) injected with these cells developed metastatic disease. Evaluation of signal transduction pathways downstream of constitutively activated Kit L575P revealed striking activation of the phosphatidyl inositol 3-kinase (PI3K) pathway. Inhibition of the PI3K pathway pharmacologically or genetically abolished the transformation phenotypes gained by the L575P single mutant. These studies validate this Kit L575P-activated model of melanoma and establish the PI3K pathway as a dominant signaling pathway downstream of Kit in melanoma.