PIK3CD and activated PI3K-delta syndrome: Activated phosphoinositide 3-kinase delta syndrome (APDS) is a rare inborn error of immunity caused by a heterozygous gain-of-function mutation in the catalytic p110δ (PIK3CD) subunit or a heterozygous loss-of-function mutation in the regulatory p85α (PIK3R1) subunit of the phosphoinositide 3-kinase delta (PI3Kδ) pathway (1–3).