PIK3CA and breast cancer: Namely, it is an eQTL (expression quantitative trait locus) for PIK3CA (P = 0.011, Supplementary Fig. 1B) in tumors from METABRIC14, is located at its promotor region and at a DNAse I hypersensitivity site (Supplementary Fig. 1C), and is bound by POL2 in a breast cancer cell line (Supplementary Table 3).