Hemimegalencephaly, a condition in which one side of the brain is larger than the other, is also attributed to an activating PIK3CA E545K that is indistinguishable from the alteration observed in several types of malignant neoplasms, but there is no clear cancer risk in hemimegalencephaly [176, 177]. The gene discussed is PIK3CA; the disease is hemimegalencephaly.