PIK3CA, or phosphatidylinositol 3-kinase, is encoded by the PIK3CA gene. PIK3s are a family of enzymes involved in cellular functions such as cell growth, proliferation, differentiation, motility, survival, and intracellular trafficking which in turn are involved in cancer. PIK3s comprise an 85 kDa regulatory subunit and a 110 kDa catalytic subunit. PIK3CA is the catalytic subunit that uses ATP to phosphorylate phosphatidylinositol. PIK3CA, as well as KRAS, regulates signaling pathways downstream of the HER-family receptors. PIK3 signaling is normally inhibited by PTEN or phosphatase and tensin homolog.