One interesting study, in which only 6% CRC patients showed evaluated CEA mRNA in CTCs while 38% CRC patients showed at least 100-fold increased CK20 using real time quantified PCR, suggested that CEA expression on CTCs may be negative or of low level, implying that CEA−/lo CTCs may play an important role in metastasis [49]. This evidence concerns the gene CEACAM5 and colorectal carcinoma.