PIK3CD and Down syndrome: These diseases include PHTS, TS (resulting from inactivating mutations in the mTORC1‐suppressing TSC1/2 proteins) [14, 15], PIK3CA‐related overgrowth syndrome (PROS; resulting from mosaic activating mutations in PIK3CA, the gene encoding the p110α PI3K catalytic subunit) [53], and activated PI3Kδ syndrome (APDS; due to germline activating mutations in PIK3CD, the gene encoding the p110δ PI3K catalytic subunit) [54] (reviewed in ref 55).