CYLD inhibits Tax-stimulated activation of IKK but not that of Tak1. HEK293 cells were transfected with the indicated expression vectors. Cell lysates were subjected to in vitro kinase assays to detect the activation of endogenous Tak1 (panel 1) and IKK (panel 3). Following autoradiography, the kinase assay membranes were subjected to IB to detect the Tak1 (panel 2) and IKKβ (panel 4) proteins. The expression level of CYLD and Tax was monitored by IB (panels 5 and 6).