PIK3CA and breast cancer: Taken together, the analysis of all somatic CNAs and mutations in HER2 heterogeneous breast cancers not only revealed the founder genetic events (that is, somatic genetic alterations likely present in all cells) of these tumors, including TP53 and PIK3CA mutations, but also led to the identification of at least one potential alternative driver genetic alteration restricted to the HER2-negative component in the breast cancers analyzed (Table 1).