ZAP70 and Down syndrome: Although this topic is extremely important in the literature, one can find only a dozen or so articles discussing this issue in various diseases classified as PID, including: activated phosphatidylinositide 3-kinase delta (PI3Kδ) syndrome (APDS) [35,36,37]; LPS-responsive beige-like anchor (LRBA) deficiency [38,39]; autoimmune lymphoproliferative syndrome (ALPS) [40,41,42]; familial HLH (FHL) [43]; ZAP70 deficiency [44]; tyrosine kinase 2 (TYK2) [45]; and Wiskott–Aldrich syndrome (WAS) [46,47,48].