For this purpose, palbociclib-resistant PIK3CA-mutant breast cancer PDXs were established from primary breast tumor (HBCx-86; PIK3CA-E545K) or from metastatic bone biopsies of patients who had progressed on ET combined with palbociclib (HBCx-180; PIK3CA-H1047R) or alpelisib (HBCx-134palboR31; PIK3CA-H1047R) [23,41]. The gene discussed is PIK3CA; the disease is breast cancer.