ESR1 and cancer: Aromatase inhibitors that include irreversible steroid inhibitors/non-steroidal inhibitors (e.g., anastrozole, letrozole, etc.)and selective ER modulators (SERMs, e.g., tamoxifen, a competitive inhibitor with equal ERα and ERβ affinity) or selective ER downregulators (SERDs, e.g., fulvestrant/ICI182780, a pure anti-ER molecule with equal affinity for ERα and ERβ which degrades both ERα and ERβ) are used against estrogen/ER-dependent cancers.