Autosomal dominant gain-of-function mutations in the PIK3CD gene encoding the catalytic subunit p110δ of phosphoinositide 3-kinase-δ (PI3K-δ) or autosomal dominant loss-of-function mutations in the PIK3R1 gene encoding the p85α, p55α and p50α regulatory subunits cause Activated PI3-kinase-δ syndrome (APDS; referred as type 1 APDS and type 2 APDS, respectively). The gene discussed is PIK3R1; the disease is Down syndrome.