To identify the molecular mechanisms by which DHX29 enhances MDA5-mediated type I IFN signaling, we tested whether DHX29 has the ability to bind to HMW Poly(I:C). To test this possibility, we performed a pull-down experiment with NeutrAvidin-beads from cell lysates of 293T cells cotransfected with biotin-labeled HMW Poly(I:C) along with MDA5, DHX29 fragments (FL, a or c), or MDA5 in combination with DHX29 fragments. We found that unlike DHX29a, FL-DHX29, DHX29c and MDA5 alone or in combination could be immunoprecipitated with anti-biotin beads (Fig 6C), suggesting that DHX29 functions as an RNA co-sensor to recognize HMW Poly(I:C). Furthermore, we performed similar experiments using LMW Poly(I:C) and found that only a small amount of DHX29 could be immunoprecipitated with anti-biotin beads (S6C Fig), suggesting that DHX29 preferentially binds to HMW Poly(I:C).