ESR1 and breast cancer: ER+ disease is one of the most common types of breast cancer, accounting for nearly 70–75% of all cases.6 There are two different molecular forms of the ER, which are ERα and ERβ, coded by different genes, and their expression patterns differ.7 The ER pathway is targeted by endocrine therapies that either repress ER functions or deplete the ligand estrogen, its ligand.