IntroductionSevere combined immunodeficiency (SCID) involves a failure of T cells to proliferate to various stimuli and a failure of B cells to produce specific antibodies. 1,2 Adenosine deaminase (ADA) deficiency, an autosomal recessive genetic defect, produces a SCID phenotype. 3,4 The enzyme defect leads to an abnormality in purine nucleoside metabolism that interferes with lymphocyte viability and function. ADA-deficient SCID is characterized by growth delay, candidiasis, respiratory infections, opportunistic infections, and, without specific therapy, early death. 5 Low or absent ADA activity in erythrocytes or other cells is diagnostic.Bone marrow transplantation (BMT) is effective therapy for SCID. The profound T-cell dysfunction associated with ADAdeficient SCID prevents graft rejection and permits BMT without conditioning with an HLA-matched sibling donor. 1,2,6 Success rates are greater than 90%, with full recovery of T-cell and B-cell functions. 6 Complete T-cell engraftment is routinely demonstrated, and approximately 50% of patients demonstrate engraftment of donor B cells. 7 Without conditioning, host hematopoiesis persists with no evidence of myeloid or erythroid engraftment. 5,[7][8][9] Study design J.L., a 17-month-old girl, presented with fever, vomiting, cough, skin ulcers, a history of failure to thrive, recalcitrant thrush, chronic respiratory infections, and recurrent otitis media. Immunizations were current. Family history was negative for immunodeficiency. Physical examination revealed an ill-appearing white female with weight and height below the fifth percentile, a paucity of lymph nodes, and no hepatosplenomegaly.Laboratory evaluation revealed normal hemoglobin and platelet counts. The white blood cell count was 1600/cmm (normal, 6000-17 000/cmm) and consisted of 10% neutrophils, 20% lymphocytes, 65% monocytes, and 4% eosinophils. Liver function test results were normal. Titers were negative for human immunodeficiency virus, cytomegalovirus, hepatitis B virus, herpes simplex virus, Epstein-Barr virus, adenovirus, poliovirus, Toxoplasma, and Chlamydia trachomatis.ADA activity was deficient in erythrocytes and in peripheral blood mononuclear cells; purine nucleoside phosphorylase (PNP) activity was normal in both cell types (Table 1). Metabolic findings in erythrocytes were consistent with ADA deficiency, including elevated deoxyadenosine nucleotide (dAXP) of 0.491 mol/mL packed cells (normal, less than 0.002 M/mL), and reduced S-adenosylhomocysteine hydrolase activity of 0.42 nmol/h/mg protein (normal, 4.2 Ϯ 1.9). 3 A 3-year-old sister was HLA-identical (A2, A3, B35, B27, Cw2, Cw4, DRB1*01, and DRB1*04). Without myeloablative or immunosuppressive therapy, the patient was infused with 7.6 ϫ 10 8 unfractionated nucleated marrow cells per kilogram from her sister. After BMT, the patient engrafted rapidly, remained well, and required no blood products (Figure 1). Biopsy-proven grade I skin graft-versus-host disease (GVHD) developed at day 39 after BMT and resolved with topical corticosteroid...