Based on this very encouraging preclinical and early clinical data with AZD9833 a phase 2 study comparing the efficacy and safety of AZD9833 versus fulvestrant (SERENA-2; NCT04214288), a presurgical “window of opportunity” study (SERENA-3; NCT04588298) and a phase 3 study comparing the effects of AZD9833 and palbociclib versus anastrozole and palbociclib as an initial treatment for women with ER+ HER2– advanced BC (SERENA-4; NCT04711252) have commenced and form part of a comprehensive development program for this agent. This evidence concerns the gene ESR1 and breast cancer.