Adenosine deaminase deficiency, a form of severe combined immunodeficiency (SCID), known as the T(−)B(−)NK(−) form of SCID, is a disorder of the purine salvage pathway that causes profound lymphopenia and molecular defects that lead to severe compromise in the number and function of T cells, of B cells, and occasionally of natural killer (NK) cells [16–18]. This evidence concerns the gene ADA and lymphopenia.