Gene expression profiling, as well as immuno-histochemical analysis of estrogen receptor (ER) α, progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2) can classify human breast cancers in four major histopatological/therapeutical subtypes: luminal A, luminal B, HER2 and basal-like [1, 2]. The gene discussed is ESR1; the disease is breast cancer.