A surrogate classification based on histological (hormone receptor expression) and molecular (Ki67 proliferation marker index) features is used in clinics, dividing breast cancers into five subtypes: luminal A-like, luminal B-like HER2-, luminal B-like HER2+, HER2 enriched, and triple-negative (Harbeck et al., 2019). The gene discussed is ERBB2; the disease is breast cancer.