A novel gene (designated 103P2D6) and its encoded protein are described. 103P2D6 is not expressed in normal adult tissue, but is highly expressed in prostate tissue xenografts, providing evidence that it is turned on in prostate cancer. 103P2D6 is also expressed in some fetal tissues, and in breast, bladder, lung, bone, colon, pancreatic, testicular, cervical and ovarian cancers. Consequently, 103P2D6 provides a diagnostic and/or therapeutic target for cancers, and the 103P2D6 gene or fragment thereof, or its encoded protein or a fragment thereof can be used to elicit an immune response.