Interestingly, Wallwiener et al. conducted a retrospective study and compared HER2 expression in primary tumors, metastatic tissue and CTCs from 107 MBC patients which revealed that primary and/or metastatic tumor tissue HER2-negative may CTCs HER2-positive, HER2 status can change during the course of breast cancer [13]. Here, ERBB2 is linked to metastatic neoplasm.