As the HER2 heterogeneity observed by scWB in this first cohort of clinical breast tumor samples exceeds our previously established scWB intra-assay technical variation threshold of 32% CV,39 we attribute the measured HER2 variation to biological differences between cells (CVt0909 = 65%; CVt0903 = 111%; CVt0119 = 44%; CVt0301 = 52%). This evidence concerns the gene ERBB2 and breast neoplasm.