Activated phosphoinositide 3-kinase δ syndrome (APDS) is an autosomal dominant primary immunodeficiency caused by gain-of-function (GOF) mutations in PIK3CD,1, 2 which encodes the p110δ catalytic subunit of phosphoinositide 3-kinase δ (PI3Kδ). The gene discussed is PIK3CD; the disease is activated PI3K-delta syndrome.