Based on molecular features (e.g., expression of estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), and the proliferation marker Ki-67) BC is divided into different molecular subtypes, with the hormone receptor positive (ER and PR+) being by far the most common subtype (60–80% of all BC cases) [2]. The gene discussed is ESR1; the disease is breast cancer.