Markers based on circulating carcinoembryonic antigen (CEA) levels and various tumour-associated gene mutations including microsatellite instability (MSI), loss of heterozygosity of 18q, deleted in colorectal cancer (DCC), mutations in KRAS, BRAF and PIK3CA genes have all been shown to be of some prognostic or predictive value (reviewed in [8,10]). The gene discussed is CEACAM5; the disease is cancer.