CEACAM5 and cancer: At the same time, the most clinically useful cancer biomarkers are low-abundance molecules; like prostate specific antigen (PSA) (Wang et al. 1979; Stamey et al. 1987), members of the mucin family of proteins (for example, CA 15.3), carcinoembryonic antigen (CEA) and cytokeratins (i.e. TPA, TPS and Cyfra 21.1; Seregni 2004) being examples.