RAG2 and Omenn syndrome: Mis-sense mutations in rag1 and rag2 are the cause of Omenn syndrome, a disease with graft-versus-host disease-like clinical presentation.8,9 Omenn syndrome is characterized by expansion of autoreactive CD4+ T cells with an oligoclonal repertoire and is fatal to infants between 2 and 6 months of age as a result of recurrent infections.10