JAK3 and severe combined immunodeficiency: The X‐linked SCID (X‐SCID), caused by mutations in IL2RG gene encoding the common gamma‐chain (γ‐chain), is the most frequent form of the disease and its clinical phenotype is comparable to that of autosomal recessive SCID (AR‐SCID) caused by Janus kinase 3 (JAK3) gene mutations (OMIM: * 600173).