Activated phosphoinositide 3‐kinase (PI3K) δ syndrome (APDS) is caused by an autosomal dominant gain-of-function mutation in phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit δ (PIK3CD), which encodes PI3Kδ and is responsible for downstream intracellular signaling in leukocytes [1]. Here, PIK3CD is linked to Down syndrome.