Saturation of the target on CEA-positive BxPC3 and LS174T cells by adding a large excess of unlabeled proteins brought down the activity bound to CEA-expressing cancer cells to background levels (144 ± 77 and 710 ± 160 CPM/105 cells, n = 3), whereas the value for the CEA-negative HT29 remained at background level (129 ± 19) (Fig. S15). Here, CEACAM5 is linked to cancer.