PIK3CD and activated PI3K-delta syndrome: APDS is caused by a gain of function mutation in the PIK3CD gene that encodes for catalytic subunit (p110δ) of PI3Kδ [APDS 1] or loss of function mutations in PIK3R1 gene that encodes for regulatory subunit (p85α) of PI3Kδ [APDS2] (82).