PIK3CA and breast cancer: It has been proposed that PIK3CA has a role in tumor initiation, as its mutations are detected in precursors of breast cancer.28 To investigate the role of myr-p110α in tumor initiation and progression, we exploited the Cre-inducible oncogenic mouse model p53fl/fl;KrasG12D, which harbors a homozygous p53 deletion and a KrasG12D activating missense allele that rapidly grows tumors.29 Based on the published breast cancer genomics data, KrasG12D mutation tends to co-occur with p53 mutations (Supplementary Figures 3A and 3B).