A novel gene 191P4D12(b) and its encoded protein, and variants thereof, are described wherein 191P4D12(b) exhibits tissue specific expression in normal adult tissue, and is aberrantly expressed in the cancers listed in Table I. Consequently, 191P4D12(b) provides a diagnostic, prognostic, prophylactic and/or therapeutic target for cancer. The 191P4D12(b) 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 191P4D12(b) can be used in active or passive immunization.