Antibodies and molecules derived therefrom that bind to 24P4C12 protein and variants thereof, are described wherein 24P4C12 exhibits tissue specific expression in normal adult tissue, and is aberrantly expressed in the cancers listed in Table I. Consequently, 24P4C12 provides a diagnostic, prognostic, prophylactic and/or therapeutic target for cancer. The 24P4C12 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 24P4C12 can be used in active or passive immunization.