ERBB2 and breast cancer: Moreover, the correlation of breast cancer differentiation state with tumor behavior and patient outcome can also be gleaned from the current classification schemes of breast cancer whether based on evaluating the histological expression of the estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor-2 (HER2), or classifications based on intrinsic gene expression and genomic profiling (PAM 50) (14, 15).