ERBB2 and breast cancer: HER2-WT mRNA expression significantly differed between cell lines classified as HER2 −, which is expected as all breast cancer cell lines are heterogeneous in HER2 expression (MCF-7 vs T47D p < 0.0159, MCF-7 vs MDA-MB-231 p < 0.0001, T47D vs MDA-MB-231 p < 0.0001).