In this regard, clinical studies of PI3K inhibitors that focus on the subset of ER+ patients with either tumors of the luminal B molecular subtype (which could be defined, for example, by using the PAM50 clinical assay [57]) or a prediction of poor outcome on hormone therapy alone (for example, as defined by using the Genomic Health Index [58]), could provide valuable information on targeting the PI3K pathway in breast cancer. Here, ESR1 is linked to breast cancer.