Employing the candidate genetic approach, we previously identified mutations in the gene encoding NEMO (NF- kB essential modulator), an intracellular signaling constituent of the NF-kB pathway, results in ectodermal dysplasia with an immune deficiency. Mutations in the zinc finger domain of NEMO block CD40 mediated activation of NF- kB and prevent B cells from undergoing class switch recombination (CSR) and APCs from synthesizing NF -kB regulated cytokines such as IL -12 or TNF-a when stimulated with CD40 ligand. Interestingly, NF- KB activation via other signaling pathways such as TNF-a or members of the Toll- like receptor family are intact in these patients. [unreadable] [unreadable] Ectodermal dysplasisa with immune deficiency has also been associated with a single heterozygous mutation at position Ser32 of the NF-kB inhibitor IkBa, one of two phosphorylation sites that are essential for targeting IkBa for proteosomal degradation and hence for activation of NF-kB. We reported a novel heterozygous nonsense mutation in the IKBA gene of a one-year-old male child with EDI that introduces a premature STOP codon at position Glu14. An in-frame methionine downstream of the nonsense mutation allows for reinitiation of translation. The resulting N-terminally truncated protein lacks both serine phosphorylation sites and inhibits NF-kB signalling by functioning as a dominant negative on NF-kB activity in lymphocytes and monocytes. These findings support the scanning model for translation initiation in eukaryotes and confirm the critical role of the NF-kB in the human immune response. [unreadable] [unreadable] CYLD is a deubiquitinating enzyme that targets signaling constituents of the NF-kB signaling pathway, including NEMO. Alteration in CYLD have been described in patients with familial cylindromatosis, a condition characterized by numerous benign adnexal tumors. In mice deficient deficient in CYLD we previously showed that the development of B cells, T cells, and myeloid cells is unaffected in CYLD deficient mice, but that the activation of these cells with mediators of innate and adaptive immunity results in enhanced NF-kB activity and is associated with increased NEMO ubiquitination. CYLD deficient mice are more susceptible to induced colonic inflammation and show a dramatic increase in the incidence of tumors in a colitis associated cancer (CAC) model. These results suggest that CYLD limits inflammation and tumorigenesis by regulating NEMO ubiquitination in vivo. We are now studying the regulatory role of CYLD in other signaling pathways and are making a CYLD knockout NEMO knockin mice.