The expression of FOXM1 was markedly decreased in the ER+PIK3CA-mutant PI3Kα inhibitor-sensitive breast tumor models (HCI-011, MCF7, T47D) following PI3Kα inhibition, whereas its expression was unchanged in the PIK3CA-wt tumor models (MDAMB231, MDAMB468, HCI-001) that were resistant to PI3Kα inhibitors. The gene discussed is FOXM1; the disease is neoplasm.