Severe combined immunodeficiency (SCID) is the most serious primary or congenital human immunodeficiency disorder. It is a group of congenital disorders in which the immune system does not work properly. Children with SCID are vulnerable to recurrent severe infections, retarded growth, and early death. X-linked SCID occurs primarily in males, who appear normal at birth due to the transfer of maternal antibodies through the placenta before birth. Around 3-6 months of age, as those levels decrease and the infant's body must begin to make its own antibodies, the number of bacterial, viral and fungal infections starts to increase. The severity and length of time for each infection increase as well. Affected infants typically do not grow at normal rates the first year of life and often are missing their tonsils and lymph nodes. Additional symptoms include skin rashes and chronic diarrhea. Without treatment, death typically occurs within the first year of life. Bone marrow or cord blood transplant is the only known cure for X-linked SCID and outcome is better if the transplant happens as soon as a diagnosis is made. X-linked SCID is caused by pathogenic variants in the IL2RG gene.