Activated phosphoinositide 3-kinase δ syndrome (APDS) is a novel autosomal-dominant primary immunodeficiency (PID) caused by a heterozygous gain-of-function mutation in the PIK3CD gene encoding the p110δ protein, the catalytic subunit of phosphoinositide 3-kinase δ (PI3Kδ).1 The c.3061G>A mutation results in a substitution of a glutamic acid by a lysine at position 1021 (E1021K). This new PID is characterized by recurrent respiratory infections, leading to bronchiectasis, progressive lymphopenia, and defective antibody production.