PIK3CD and activated PI3K-delta syndrome: APDS is caused by gain of function (GOF) variants in the PIK3CD gene/p110δ protein (APDS1) or loss of function (LOF) mutations in the PIK3R1 gene/p85α protein (APDS2), which constitute the subunits of the PI3Kδ kinase, a protein complex that is essential for the maturation of T and B lymphocytes and the regulation of their activity (9, 13, 14).