A tumor biomarker, e.g., alpha-fetoprotein (AFP), carcinoembryonic antigen (CEA), cancer antigen 125 (CA 125), or cancer antigen 15-3 (CA 15-3), is anything present in, or produced by, cancer cells in the body, relating to cancer or certain benign (noncancerous) conditions [1,2,3]. Here, MUC1 is linked to cancer.