In a phase II trial of alisertib monotherapy at the RP2D, alisertib was associated with a 6-month clinical benefit rate (CBR = CR + PR + SD for ≥ 6 months) of 54% and median progression-free survival (PFS) of 7.9 months in those with heavily pre-treated, ER+/HER2-negative breast cancer (n = 26) [26]. The gene discussed is ESR1; the disease is breast cancer.