Adenosine deaminase (ADA) deficiency is an autosomal recessive disorder resulting in immunodeficiency. Since the cDNA for ADA was cloned approximately 10 years ago, investigators have determined the molecular basis for disease in many patients with ADA deficiency. Mutations that have been identified include point mutations causing amino acid substitutions, premature stop codons, RNA splicing errors, and deletion mutations. Approximately one third of patients are homozygous for their mutation; in some of these cases the parents are known to be related. One mutation, Ala329-Val, is the most common, being present in 8 of the 21 ADA-deficient SCID patients whose mutations have been reported.