PIK3R1 and breast cancer: Examining properties of the cell lines in each cohort, it is notable that the PI3K-predicted subset was relatively enriched in breast cancers (55% vs. 0%), and PI3K pathway mutations (PIK3CA, PTEN, or PIK3R1; 64% vs. 44%) as compared to the MAPK-predicted subset (Fig 4B), consistent with the characteristics of our internal 18 cell line panel.