ERBB2 and breast cancer: In the case of MCF-7 and SKBR-3 breast cancer cell lines, Xiao et al., observed that MCF-7 cells have a typical number of gene copies (i.e., 2) and very low HER2 protein content, whereas SKBR-3 cells contain approximately 10-fold more copies of the HER2 gene and 15-fold higher HER2 protein content [27].