ESR1 and breast cancer: To model the hormonal milieu of ER-mutant BC in women on AI therapy, we performed prolonged (>3 months) culture of CRISPR heterozygous knock-in ER Y537S or D538G MCF7 and T47D cells (13) in phenol red-free media with charcoal-stripped FBS and 1nM dihydrotestosterone (DHT).