ESR1 and neoplasm: Alisertib’s safety and tolerability profile was well-defined in early phase trials in hematologic cancers.10,11,12 In a phase 2 basket trial of alisertib monotherapy for solid tumors, among those with heavily pretreated, ER+/ERBB2− (formerly HER2) metastatic breast cancer (MBC) (n = 26), a 23% objective tumor response rate (ORR), 6-month clinical benefit rate (CBR) of 54%, and median progression-free survival (mPFS) of 7.9 months were observed.13