ERBB2 and breast cancer: Breast cancer can be divided into four different subgroups dependent on immunohistochemistry: hormone receptor (estrogen/progesterone) positive, human epidermal growth factor-2 (Her2) negative breast cancer with low Ki67 < 25 (Luminal A) and high Ki67 ≥ 25 (Luminal B); Her2 positive tumors; and triple negative (estrogen, progesterone and Her2 negative) breast cancer [21].