Abstract: This invention relates to isolated proteins and to peptides which are found on the surface of colon cells and colon cancer cells, as well as to nucleic acid molecules encoding said protein and peptides. The protein and peptides bind to tumor associated antibodies, such as mAb 33. The monomeric protein has a molecular weight of about 43 kD as determined by SDS gel electrophoresis under non-reducing conditions. In addition, this invention relates to the use of said nucleic acid molecules, protein, in monomeric or multimeric form, and to antibodies to said peptides in diagnostic, screening and therapeutic methods. This invention further relates to antibodies specific for said protein, in monomeric or multimeric form, and to antibodies to said peptides.