ERBB2 and breast cancer: The breast cancer cell line SK-BR-3 has increased expression of HER2 (with approximately 1.5 × 106 HER2 protein molecules per cell, compared to ∼4.0 × 104 and ∼2 × 104 in HER2-negative cell lines MDA-MB-231 and A431, respectively (Sato et al., 2013; Shaban et al., 2024).