The transcription factor NF-κB is required for lymphocyte activation and proliferation as well as the survival of certain lymphoma types^,. Antigen receptor stimulation assembles an NF-κB activating platform containing the scaffold protein CARMA1 (also called CARD11), the adaptor BCL10 and the paracaspase MALT1 (the CBM complex), linked to the inhibitor of NF-κB kinase complex^,,,,,,,,,, but signal transduction is not fully understood. We conducted parallel screens involving a mass spectrometry analysis of CARMA1 binding partners and an RNA interference screen for growth inhibition of the CBM-dependent ‘activated B-cell-like’ (ABC) subtype of diffuse large B-cell lymphoma (DLBCL). Here we report that both screens identified casein kinase 1α (CK1α) as a bifunctional regulator of NF-κB. CK1α dynamically associates with the CBM complex on T-cell-receptor (TCR) engagement to participate in cytokine production and lymphocyte proliferation. However, CK1α kinase activity has a contrasting role by subsequently promoting the phosphorylation and inactivation of CARMA1. CK1α has thus a dual ‘gating’ function which first promotes and then terminates receptor-induced NF-κB. ABC DLBCL cells required CK1α for constitutive NF-κB activity, indicating that CK1α functions as a conditionally essential malignancy gene—a member of a new class of potential cancer therapeutic targets.