Hyper-immunoglobulin M (IgM) syndrome (HIGM) is a rare primary immunodeficiency characterized by elevated or normal IgM and absent or markedly decreased amounts of IgG, IgA and IgE. The X-linked form (HIGM1) is the most common type and is caused by mutations in the gene for CD40L, a T-cell surface molecule required for T-cell driven immunoglobulin class switching by B cells. In the present study we have identified a patient with X-linked hyper-IgM who failed to express CD40L on the cell surface of CD4(+) T lymphocytes. Sequence analysis of CD40L genomic DNA showed no mutations. CD40L mRNA was absent and sequence analysis of the CD40L promotor revealed a mutation at position -123 from the transcription start site. The mutation in the promotor region likely contributed to the decreased transcription as demonstrated by a luciferase reporter assay.