RIG-I/MDA5 plays a pivotal role in innate immunity by detecting intracellular double-stranded RNA (dsRNA) and activating the transcription of type I interferons and proinflammatory factors, but the exactly regulating mechanism of RIG-I/MDA5 signaling remains elusive. In this study, UbL-UBA domain containing protein RAD23A was identified as a negative regulator of RIG-I/MDA5-mediated signaling activation through a small interfering RNA (siRNA)-based screening. Knockdown of RAD23A augmented the expression of RIG-I/MDA5-mediated expression of proinflammatory cytokines and IFN-beta whereas ectopic expression of RAD23A showed the converse effect. Moreover, we confirmed the interaction between RAD23A and tumor necrosis factor receptor-associated factor 2 (TRAF2), an essential mediator of RIG-I/MDA5 signaling, and found that RAD23A down-regulated TRAF2 protein level through ubiquitin-proteasome system. Therefore, this study identified RAD23A as a novel negative regulator of RIG-I/MDA5 mediated anti-virus response. (C) 2013 Elsevier Inc. All rights reserved.