CEACAM5 and cancer: For these reasons, peripheral blood is an optimal source for cancer biomarkers, including proteins [such as carcinoembryonic antigen (CEA) and cancer antigen 19-9 (CA 19-9)], exosomes, and cell-free tumor DNA (cfDNA)—all which derive from cancer cells [10,11,12,13,14,15,16]—as well as intact disseminated tumor-associated cell populations [17,18,19,20,21,22,23,24,25,26,27,28,29].