A randomized double-blind phase II trial in ER-positive, HER2-negative metastatic breast cancer that had been resistant to treatment with aromatase inhibitor indicated that therapy efficiency did not benefit from the addition of the pan-class I PI3K inhibitor pictilisib (Genentech; GDC-0941) to fulvestrant (an ER antagonist) (Krop et al., 2016). This evidence concerns the gene CYP19A1 and breast cancer.