Tumor markers have been established as useful indicators which aid in tumor diagnosis or prognosis (e.g., alpha-fetoprotein (AFP) in hepatocellular carcinoma (HCC) [1,2,3,4]), facilitate treatment decisions (e.g., human epidermal growth receptor 2 in breast cancer (reviewed in [5])), and monitor disease progression and the effectiveness of treatment (e.g., carcinoembryonic antigen (CEA) in colorectal cancer (CRC) (reviewed in [6,7,8])). Here, CEACAM5 is linked to breast cancer.