ESR1 and neoplasm: In the PALOMA-3 trial in which patients with HR+ MBC who progressed on prior AI were randomized to fulvestrant with or without palbociclib, there was a numerically lower PFS among patients with ESR1-mutant tumor compared to those with ESR1 WT (3.6 vs. 5.4 months) in the fulvestrant arm, but statistical significance was not reached [13].