The most frequent form of SCID is X linked SCID or SCID-X1. Affecting nearly 45% of all cases of SCID is due to a mutation in the gene IL2RG on the X chromosome.



Mutations on this gene result in very low ​T-lymphocyte and NK-lymphocyte counts, but the B-lymphocyte count is high (a so-called T-, B+, NK-phenotype) because this mutation produce a lack of the common γc cytokine receptor subunit, which causes a complete block in T-cell and natural killer (NK) cell development.



Despite the high number of B-lymphocytes, there is no B-lymphocyte function since the T-cells are not able to “help” the B-cells to function normally. Without the normal function of T cells, B cells and NK cells the immune system can not afford the important process of the organism defence and humans with this disease are susceptibility to die in front of any simple pathogen exposition. This deficiency is inherited as an X-linked recessive trait. Only males have this type of SCID, but females may carry the gene and have a 1 in 2 chance (50%) of passing it on to each son.

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To summarize, SCID-X1 is a lethal condition that can be cured by allogeneic stem-cell transplantation but, what happen if there is no compatible donor for this patient? Researchers and different medical investigation centres are working hard since approximately 20 years till today on many clinical trials and investigations about infusion of autologous hematopoietic stem cells that had been transduced in vitro with the γc gene can restore the immune system in patients with SCID, this is what researchers call ​gene therapy. (See Gene Therapy page for better comprehension).