PIK3CA and breast cancer: In vitro and xenograft models of breast cancer have also demonstrated that cells harboring amplification of the RTK HER2 are dependent on PI3K pathway activation and sensitive to its inhibition through targeting of PI3K (O’Brien et al., 2010; Tanaka et al., 2011), dual PI3K/mTOR (Brachmann et al., 2009), AKT (She et al., 2008), and mTOR kinase (Weigelt et al., 2011).