ESR1 and neoplasm: In addition to MCF-7 model, the activity of ZN-c5 in ER-mutant models including WHIM20, a Y537S ESR1 patient-derived xenograft model, was evaluated, resulting in an induced 64% tumor growth inhibition at 40 mg/kg dosing while fulvestrant at 200 mg/kg (eightfold higher than that achieved in the clinic) resulted in 13% tumor growth inhibition.