The pathogenesis of mucosa-associated lymphoid tissue (MALT) lymphomas is associated with independent chromosomal translocations that lead to the upregulation of either BCL10 or MALT1 or the generation of a fusion protein, cIAP2-MALT1. While both BCL10 and MALT1 are critically involved in antigen receptor-mediated NF-kappaB activation, the role of cIAP2 is not clear. Here we show that cIAP2 is a ubiquitin ligase (E3) of BCL10 and targets it for degradation, inhibiting antigen receptor-mediated cytokine production. cIAP2-MALT1 lacks E3 activity, and concomitantly, the BCL10 protein is stabilized in MALT lymphomas harboring this fusion. Furthermore, BCL10 and cIAP2-MALT1 synergistically activate NF-kappaB. These results reveal cIAP2 as an inhibitor of antigenic signaling and implicate its dysfunction in MALT lymphomas.