PIK3CA and breast cancer: Not only genomic predictors but also PI3K pathway activation state as determined by expression levels of markers upstream of mTORC1, such as p-AKT, or downstream of mTORC1, such as p-S6K (Figure 1), have been shown to correlate with sensitivity to allosteric mTOR inhibitors in breast cancer cell lines in vitro (Noh et al., 2004; Meric-Bernstam et al., 2012).