To extend our analysis to breast cancer PDX models harboring mutations that are commonly acquired during course of therapy, we tested the activity of elacestrant in an endocrine and palbociclib-resistant model of ER+/HER2- breast cancer (CTG-1260), which has two mutations in PIK3CA, D350H, and H1047R, and the D538G mutation in ESR1. CTG-1260 was derived from a patient on fulvestrant for 9 months and initially responded. The gene discussed is ESR1; the disease is breast cancer.