PURPOSEIdentification of novel biomarkers and immunotherapy targets for prostate cancer (PCa) is crucial to better diagnosis and therapy. We sought to identify novel PCa tumor-associated antigens (TAA) that are expressed in PCa, absent in nonprostate human tissue, and immunogenic for immune responses restricted by human HLA. Experimental Design andRESULTSUsing microarray analysis of normal and cancerous human prostate tissues, we identified 1,063 genes overexpressed in PCa. After validating 195 transcripts in publicly available array data sets, we interrogated expression of these TAAs in normal human tissues to identify genes that are not expressed at detectable levels in normal, nonprostate adult human tissue. We identified 23 PCa TAA candidates. Real-time PCR confirmed that 15 of these genes were overexpressed in PCa (P< 0.05 for each). The most frequently overexpressed gene, single-minded homologue 2 (SIM2), was selected for further evaluation as a potential target for immunotherapy. ELISA assay revealed that a fraction of PCa patients exhibited immune responsiveness to SIM2 as evidenced by the presence of autoantibodies to SIM2 in their sera. We next showed binding of putative HLA-A2.1-restricted SIM2 epitopes to human A2.1, and immunization of transgenic HLA-A2.1 mice showed induction of SIM2-specific CTL responses in vivo.CONCLUSIONSOur findings that SIM2 is selectively expressed in PCa, that human HLA-A2.1-restricted SIM2 epitopes induce specific T cells in vivo, and that anti-SIM2 antibodies are detectable in PCa patients' sera implicate SIM2 as a PCa-associated antigen that is a suitable potential target for PCa immunotherapy.