Synucleins are a family of highly conserved small proteins predominantly expressed in neurons. Recently we and others have found that ␥-synuclein is dramatically up-regulated in the vast majority of late-stage breast and ovarian cancers and that ␥-synuclein over-expression can enhance tumorigenicity. In the current study, we have found that ␥-synuclein is associated with two major mitogen-activated kinases (MAPKs), i.e. extracellular signal-regulated protein kinases (ERK1/2) and cJun N-terminal kinase 1 (JNK1), and have shown that over-expression of ␥-synuclein leads to constitutive activation of ERK1/2 and down-regulation of JNK1 in response to a host of environmental stress signals, including UV, arsenate, and heat shock. We also tested the effects of ␥-synuclein on apoptosis and activation of JNK and ERK in response to several chemotherapy drugs. We have found that ␥-synuclein-expressing cells are significantly more resistant to the chemotherapeutic drugs paclitaxel and vinblastine as compared with the parental cells. The resistance to paclitaxel can be partially obliterated when ERK activity is inhibited using a MEK1/2 inhibitor. Activation of JNK and its downstream caspase-3 by paclitaxel or vinblastine is significantly down-regulated in ␥-synuclein-expressing cells, indicating that the paclitaxel-or vinblastine-activated apoptosis pathway is blocked by ␥-synuclein. In contrast to paclitaxel and vinblastine, etoposide does not activate JNK, and ␥-synuclein over-expression has no apparent effect on this drug-induced apoptosis. Taken together, our data indicate that oncogenic activation of ␥-synuclein contributes to the development of breast and ovarian cancer by promoting tumor cell survival under adverse conditions and by providing resistance to certain chemotherapeutic drugs.Breast carcinoma is the second leading cause of cancer-related deaths in women of the Western world. In the United States alone over 180,000 new cases are diagnosed annually and more than 40,000 women die from this disease each year (1). Epithelial ovarian cancer continues to be the leading cause of death from gynecologic malignancies in the United States (1, 2). One woman in 70 in the U.S. will develop ovarian cancer in her lifetime, and one woman in 100 will die of this disease. Breast and ovarian cancer etiology are multifactorial, involving environmental factors, hormones, genetic susceptibility, and genetic changes during progression. Both cancers are a heterogeneous group of tumors with no unifying molecular alteration yet identified. A certain number of breast and ovarian cancer cases (ϳ5-10%) are attributed to inherited mutations in highly penetrant breast cancer susceptibility genes, such as BRCA1 and BRCA2 (reviewed in Ref. 3). However, the majority of the tumors occur in women with little or no family history, and the molecular basis of these sporadic cancers is still poorly defined.In an effort to identify other genes involved in the development and/or progression of breast and ovarian cancer, we and others used differentia...