CEACAM5 and breast cancer: Compared with classical biomarkers of cancer antigen (CA) 15-3 [7], carcinoembryonic antigen (CEA) [8], HER2 protein [9], microRNA-21 [10], microRNA-195 [11], and microRNA-125b [12], exosomes have emerged as a promising non-invasive biomarker for the early diagnosis and treatment of breast cancer [13].