The cylindromatosis tumor suppressor gene (Cyld) encodes a deubiquitinating 
enzyme (CYLD) with immunoregulatory function. In this study, we evaluated the 
role of Cyld in T cell ontogeny by generating a mouse (Cyld(Delta9)) with a 
thymocyte-restricted Cyld mutation that causes a C-terminal truncation of the 
protein and reciprocates catalytically inactive human mutations. Mutant mice had 
dramatically reduced single positive thymocytes and a substantial loss of 
peripheral T cells. The analyses of polyclonal and TCR-restricted thymocyte 
populations possessing the mutation revealed a significant block in positive 
selection and an increased occurrence of apoptosis at the double-positive stage. 
Interestingly, in the context of MHC class I and II restricted TCR transgenes, 
lack of functional CYLD caused massive deletion of thymocytes that would have 
been positively selected, which is consistent with an impairment of positive 
selection. Biochemical analysis revealed that Cyld(Delta9) thymocytes exhibit 
abnormally elevated basal activity of NF-kappaB and JNK. Most importantly, 
inactivation of NF-kappaB essential modulator fully restored the NF-kappaB 
activity of Cyld(Delta9) thymocytes to physiologic levels and rescued their 
developmental and survival defect. This study identifies a fundamental role for 
functional CYLD in establishing the proper threshold of activation for thymocyte 
selection by a mechanism dependent on NF-kappaB essential modulator.