Positive selection occurs in the thymic cortex, but critical maturation events 
occur later in the medulla. Here we defined the precise stage at which T cells 
acquired competence to proliferate and emigrate. Transcriptome analysis of late 
gene changes suggested roles for the transcription factor NF-κB and interferon 
signaling. Mice lacking the inhibitor of NF-κB (IκB) kinase (IKK) kinase TAK1 
underwent normal positive selection but exhibited a specific block in functional 
maturation. NF-κB signaling provided protection from death mediated by the 
cytokine TNF and was required for proliferation and emigration. The interferon 
signature was independent of NF-κB; however, thymocytes deficient in the 
interferon-α (IFN-α) receptor IFN-αR showed reduced expression of the 
transcription factor STAT1 and phenotypic abnormality but were able to 
proliferate. Thus, both NF-κB and tonic interferon signals are involved in the 
final maturation of thymocytes into naive T cells.