The top five genes mutated per IEI category are shown in Fig. 2 B, with IL2RG being the most frequently reported germline genetic cause of CID; 22q11.2 deletion syndrome, of CID with syndromic features; BTK, of PAD; TNFRSF6, of category IV, diseases of immune dysregulation; CYBB, of phagocyte disorders; STAT1, of intrinsic or innate immune disorders; MEFV, of autoinflammatory syndromes; C1 inhibitor, of complement deficiencies; RTEL, of BMF; and somatic TNFRSF6, of phenocopies. The gene discussed is FAS; the disease is combined immunodeficiency.