Adenosine deaminase (ADA) deficiency in humans is one cause of severe combined immunodeficiency. When ADA fails to catalyze the debnation of adenosine and dwxyadenosine, the levels of deoxyadenosine that accumulate are toxic to lymphoid cells. Patients with complete ADA deficiency (e.g., with less than 5% normal ADA catalytic activity) lack both B-and T-lymphocyte function. Blymphoblast cell lines derived from patients with ADA deficiency have been analyzed at multiple levels. Blot hybridization and S1 nuclease analysis of ADA messenger RNA (mRNA) indicates that the majority of ADA-deficient cell lines have ADA mRNA in the same abundance and size as in normal cell lines. Sequence analysis of ADA cDNAs derived from these mRNAs shows that the majority of mutations are single base changes that alter the amino acid sequence. Expression analysis proves that these point mutations lead to deficiency of ADA catalytic activity. Several cell lines have mutations that alter mRNA transcription or processing. These include a point mutation in one allele of an ADA-deficient cell line that leads to deletion of exon 4 during mRNA splicing. In addition, two cell lines are homozygous for large deletions of the gene that are the result of homologous recombination. Subjects with partial ADA deficiency have undetectable ADA activity in their erythrocytes, variable activity in their lymphoid cells, and normal immunological function. Analysis of the ADA catalytic activity of partially deficient cell lines indicates that the mutations involved affect protein stability. However, the mutations causing partial ADA deficiency are as yet undefined.