X-linked ectodermal dysplasia and immunodeficiency (XL-EDA-ID) is an X-linked recessive disease caused by a mutation in the nuclear factor-κB (NF-κB) essential modulator (NEMO). Here we report an XL-EDA-ID patient with atypical features of very few naive-phenotype T cells and defective mitogen-induced proliferation of peripheral blood mononuclear cells (PBMCs). The patient's NEMO defect was diagnosed by flow cytometric analysis of intracellular NEMO staining. Specific cell lineages (monocytes and neutrophils) expressed reduced levels of NEMO, but 2 populations of T, B, and NK cells were detected with normal and reduced expression of NEMO. Genomic analysis revealed that duplication of a 4.4-kb sequence ranging from intron 3 to exon 6 caused the reduced expression of NEMO. Polymorphism analysis showed that the patient's B- and T-cell lines with reduced and normal expression of NEMO had the same X chromosome, indicating that the somatic mosaicism was not due to fetomaternal transfusion but was most likely due to postzygotic reversion. This XLEDA-ID case adds to our understanding of NEMO biology, indicating that NEMO is critical for T-cell development and/or survival in humans as well as in mice. (Blood. 2004;103:4565-4572)