Activated phosphoinositide 3-kinase delta syndrome (APDS), caused by mutations in PI3Kδ catalytic p110δ (PIK3CD) or regulatory p85α (PIK3R1) subunits, is a primary immunodeficiency affecting both humoral and cellular immunity, which shares some phenotypic similarities with hyper-IgM syndromes and common variable immunodeficiency (CVID). Here, PIK3R1 is linked to immunodeficiency disease.