RAG2 and severe combined immunodeficiency: Ommen syndrome, namely impaired V(D)J recombination due to mutations in RAG1 and RAG2,92, 93, 94 and defective DNA repair after V(D)J recombination by mutations in DCLRE1C/ARTEMIS95 cause SCID characterized by erythroderma, desquamation, alopecia, eosinophilia, hepatosplenomegaly, elevated serum IgE levels, and often, colitis.96 Moreover, defects in DNA ligase 4 (LIG4) encoding an ATP‐dependent DNA ligase that joins double stranded breaks during non‐homologous end joining pathway, and is essential for V(D)J recombination, can cause SCID, and can develop IBD.92