Following the discovery of PIK3CD GOF mutations, that result in constitutive p110δ activation in patients with Activated PI3K Delta Syndrome type 1 (APDS1) (51), monoallelic GOF PIK3R1 mutations were identified in 12 patients affected with a hyper–IgM-like primary immunodeficiency/immune dysregulation condition associated with T- and B-cell maturational and functional defects (52). This evidence concerns the gene PIK3CD and inborn error of immunity.