Activating mutations in the catalytic subunit of PI3-kinase (PIK3CA) occur in around 40% of ER+ breast cancers [29], but the hypothesis that PIK3CA-mutated breast cancers would derive the largest benefit from mTOR inhibitors was not confirmed in an exploratory analysis of the BOLERO-2 trial, although only a fraction of enrolled patients were included [30]. This evidence concerns the gene PIK3CA and breast cancer.