To determine potential genomic correlates with clinical benefit to buparlisib and letrozole in metastatic ER+ breast cancer patients, we performed MSK-IMPACT targeted next generation sequencing (tNGS) profiling for 341 genes using pre-study biopsy (where available) material or archival tissue from 33 patients subsequently enrolled and treated according to the study protocol. Here, ESR1 is linked to breast cancer.