Activated phosphoinositide 3-kinase (PI3K) δ Syndrome (APDS), caused byautosomal dominant mutations in PIK3CD (APDS1) orPIK3R1 (APDS2), is a heterogeneous primary immunodeficiency.While initial cohort-descriptions summarized the spectrum of clinical andimmunological manifestations, questions about long-term disease evolution andresponse to therapy remain. The gene discussed is PIK3CD; the disease is activated PI3K-delta syndrome.