PIK3CD and Immunodeficiency: Gain-of-function variants in the PIK3CD gene or loss-of-function variants in the PIK3R1 gene encoding phosphoinositide 3-kinase (PI3K) delta result in increased activation of the PI3K/AKT/mammalian target of rapamycin (mTOR)/S6K signaling pathways in immune cells, which leads to immunodeficiency and immune dysregulation [1–6].