A novel gene 251P5G2 and its encoded protein, and variants thereof, are described wherein 251P5G2 exhibits tissue specific expression in normal adult tissue, and is aberrantly expressed in the cancers listed in Table I. Consequently, 251P5G2 provides a diagnostic, prognostic, prophylactic and/or therapeutic target for cancer. The 251P5G2 gene or fragment thereof, or its encoded protein, or variants thereof, or a fragment thereof, can be used to elicit a humoral or cellular immune response; antibodies or T cells reactive with 251P5G2 can be used in active or passive immunization.