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The prominent role of the CD40 receptor in B cell responses led us to Investigate the role of the gp39-CD40 interaction in a group of primary immunodeficient patients with defective antibody production. Here we report that patients with hyper-tgM syndrome (HIM) have a defective gp39-CD40 interaction. B cells from HIM patlents express functlonal CD40, but their T cells do not bind CD40-lg. These patients expressed normal levels of gp39 mRNA, but these mRNAs encode defective gp39 proteins owing to mutations in the extracellular domain of gp39. Soluble recombinant forms of gp39 containing these mutations were unable to bind CD40 and drive normal B cell proliferation. The gene encoding gp39 was mapped to Xq26, the X chromosome region where the gene responsible for HIM had previously been mapped. These data suggest that a defect in gp39 is the basis of X-linked HIM.