Class IA PI3Ks are the most frequently implicated in breast cancer with mutation and/or amplification of genes encoding the PI3K catalytic subunits p110α (PIK3CA), p110β (PIK3CB) and p110δ (PIK3CD), and the PI3K regulatory subunit p85α (PIK3R1), etc. Of these, PIK3CA mutations are the most common genetic alterations of this pathway in breast cancer. This evidence concerns the gene PIK3CA and breast cancer.