PIK3CA (phosphatidylinositol 3-kinase, catalytic, alpha polypeptide), a common genetic driver of breast cancer, invokes mixed-lineage tumours even when expressed in unipotent progenitor cells, demonstrating plasticity (Van Keymeulen et al., 2015; Koren et al., 2015) as well as lineage switching (i.e. expression of PIK3CA in a basal-progenitor cell giving rise to luminal tumours). This evidence concerns the gene PIK3CA and breast cancer.