HER2 status of breast cancer is routinely assessed by either immunohistochemical(IHC) analysis of HER2 protein or by fluorescent in situ hybridization (FISH)analysis of ERBB2 gene copy number in primary tumor tissues.Recently it was shown that HER2 extracellular domain (ECD) can be shed into thecirculation by proteolytic cleavage from the full-length HER2 receptor, and isdetected in the serum of women with benign breast disease, primary and metastaticbreast cancer [10]. Here, ERBB2 is linked to breast cancer.