CEACAM5 and colorectal cancer: Other markers are used to characterize CTCs originating from different tumor types, including human epidermal growth factor receptor 2(Her2) and mucin 1 (MUC1) for breast cancer, prostate-specific antigen (PSA), prostate-specific membrane antigen (PSMA), androgen receptor (AR), and epidermal growth factor receptor (EGFR) for prostate cancer, and carcinoembryonic antigen (CEA) for colorectal cancer [2].