ESR1 and breast cancer: These phenomena have been observed in several in vitro studies that used experimental models of variants of BC MCF-7 cells developed from either long-term estrogen deprivation (LTED [147], E8CASS cells [149], MCF-7:5C [150]) or long-term exposure to selective ER modulators (SERM, TAM, or RLX [151,152,153,154]) which responded paradoxically to E2 with apoptosis.