The c-Jun NH2-terminal kinase (JNK) is implicated in oncogenic transformation. However, studies of the effect of Jnk gene disruption on Ras-induced transformation of murine fibroblasts indicate that JNK may act as a suppressor of Ras transformation and that the JNK signaling pathway contributes to the apoptotic elimination of transformed cells in vivo. The conclusion that JNK can act as a tumor suppressor is consistent with the presence of loss-of-function mutations in JNK pathway components (Jnk3 and Mkk4) in human tumors. Nevertheless, JNK can also contribute to the proliferation and survival responses of some tumors. A key question that remains unresolved concerns the genetic and mechanistic basis for these different roles of JNK in tumors. Indeed, an understanding of this question will be required for the rational use of small molecule inhibitors of JNK for tumor therapy.