The onconeural antigens appear to serve as tumor rejection antigens in the paraneoplastic neurologic disorders. Here, we used an unbiased peptide binding screen, followed by studies in HLA-A2.1 transgenic mice to identify naturally processed HLA-A2.1 restricted epitopes of the paraneoplastic cerebellar degeneration breast/ ovarian cancer antigen cdr2. These mice were used to clone high-avidity cdr2-specific CD8 ؉ T cells that recognize human tumor cells presenting endogenously loaded MHC class I-cdr2 peptide. T cells with this specificity were detected in the peripheral blood of two HLA-A2.1 ؉ paraneoplastic cerebellar degeneration patients. We cloned T cell receptor (TCR) ␣ and  genes from cdr2-specific T cells; electroporation of RNA encoding this TCR turned nonreactive donor T cells into efficient killers of human cdr2-expressing tumor cells. Cloned cdr2-specific TCR genes provide a clinically relevant means for immunologic targeting of human gynecologic cancers.gene therapy ͉ gynecologic cancer ͉ paraneoplastic cerebellar degeneration A new strategy for treating cancer patients has emerged from efforts to stimulate CD8 ϩ cytotoxic T lymphocytes (CTL) through vaccination or adoptive T cell transfer (1-3). This effort has relied on a growing list of human tumor antigens recognized by CTL. However, few of these tumor antigens are associated with naturally robust immune responses and spontaneous tumor rejection, and their clinical utility is unclear. The paraneoplastic neurologic disorders (PNDs), perhaps the best-known examples of naturally occurring tumor immunity (4-7), offer a different approach to this problem. For example, patients with paraneoplastic cerebellar degeneration (PCD) develop an antigen-specific and clinically robust antitumor immune response directed against their breast and ovarian carcinomas (5,8). In one series, two-thirds of PCD patients (34 of 52) presented with neurologic symptoms before the diagnosis of cancer, 87% (45 of 52) had limited oncologic disease when diagnosed, and tumor could only be found by exploratory surgery in 4 of 52; by comparison, only 50-60% of unselected breast cancer patients and 25% of ovarian cancer patients present with limited-stage disease (9).Expression of the PCD antigen, cdr2, is believed to be largely restricted to cerebellar Purkinje neurons (10). The immune system is thought to initiate PCD when cdr2 is abnormally made in breast or ovarian tumors, triggering an antitumor immune response that can break immune tolerance in the brain. Interestingly, cdr2 is expressed by a large proportion of breast (25%) and ovarian (60%) tumors from individuals who do not develop neurological disease (11). Taken together with observations of immune responses made in other PNDs (5-7), cdr2 expression, and perhaps immune responses to it, may develop independently of autoimmune responses. These observations suggest the possibility that some gynecologic cancer patients might benefit from cdr2-directed immunotherapy. In addition, the unique expression of cdr2 in tumor cells (...