Among patients with hormone receptor-positive breast cancer treated with an aromatase inhibitor plus palbociclib (Ibrance), those who displayed a rising ESR1 mutation detected in their blood before disease progression doubled their median progression-free survival following a switch to fulvestrant (Faslodex) plus palbociclib, according to results from the phase III PADA-1 clinical trial presented at the San Antonio Breast Cancer Symposium, held December 7-10, 2021.