After confirming the selectivity and specificity of the CELx HSF test, ligand-driven HER2 signals were surveyed in 10 human breast cancer cell lines overexpressing HER2 (HER2+) and 10 human breast cancer cell lines expressing lower or normal levels of HER2 (HER2-) in order to determine whether CELx HSF test positive (HSF+) and CELx HSF test negative (HSF-) populations exist among HER2+ and HER2- cell types. This evidence concerns the gene ERBB2 and breast cancer.