A novel gene 109P1D4 and its encoded protein, and variants thereof, are described wherein 109P1D4 exhibits tissue specific expression in normal adult tissue, and is aberrantly expressed in the cancers listed in Table I. Consequently, 109P1D4 provides a diagnostic, prognostic, prophylactic and/or therapeutic target for cancer. The 109P1D4 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 109P1D4 can be used in active or passive immunization.