Relative frequencies of the various molecular defects found in 174 consecutive cases of human SCID evaluated at Duke University Medical Center over the past 3 decades. The most common type is X-linked SCID, due to mutations in the gene encoding the common γ chain for multiple cytokine receptors; the second most common cause is adenosine deaminase deficiency (ADA def.), and the third most common cause is IL-7Rα–chain deficiency. In 25 cases the molecular defect remains unknown (those in the groups labeled autosomal recessive and unknown). No cases of CD45 deficiency have been seen at this institution. Def., deficiency.