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C D8 glycoproteins play an important role in both the maturation and function of MHC class I-restricted T lymphocytes. A 25-year-old man, from a consanguineous family, with recurrent bacterial infections and total absence of CD8⁺ cells, was studied. Ab deficiencies and ZAP-70 and TAP defects were ruled out. A missense mutation (gly90→ser) in both alleles of the immunoglobulin domain of the CD8α gene was shown to correlate with the absence of CD8 expression found in the patient and two sisters. Conversely, high percentages of CD4^–CD8^–TCRαβ⁺ T cells were found in the three siblings. A novel autosomal recessive immunologic defect characterized by absence of CD8⁺ cells is described. These findings may help to further understanding of the role of CD8 molecules in human immune response.