JAK3 and severe combined immunodeficiency: The prototypic example of an IEI associated with opportunistic viral infections is SCID (severe combined immunodeficiency), characterized by profound alterations of T lymphocytes and a variable absence of B and NK cells, for example, X-linked SCID (g-chain), JAK3, IL7Ra, RAG1/2, DCLRE1C/Artemis, ADA.