PIK3CA and cancer: In a panel of breast (MCF7, BT474, MDA-MB-453) and colon (HCT116) cancer cell lines, all with mutations in the gene that encodes the catalytic subunit of PI3K p110α (PIK3CA), rapamycin at 50 nM effectively inhibited phosphorylation of the mTORC1 substrate p70S6 kinase (p70S6K) and its downstream target S6 (Figure 1A).