regulators of the mTOR-signaling pathway, a second somatically acquired mutation is required for disease onset. Somatic mutations that mildly activate the mTOR-signaling pathway also cause symmetrical overgrowth syndromes such as megalencephaly-capillary malformation syndrome, megalencephaly, and certain forms of polymicrogyria (49–51). Common to all of these phenotypes is the presence of hypertrophic neural-like “balloon” cells, which carry the somatic mutation yet fail to transform to a malignant cell type (52).
