Structural features of type I, II, III, and IV picornavirus IRES elements. RNA secondary structures of four types base on M-fold software. (A). Enteroviruses (PV, CVB3 and EV71) and rhinoviruses contain type I IRES elements. (B). Aphthoviruses (FMDV) and cardiovirus (EMCV and TMEV) contain type II IRES elements. (C). HAV contains type III IRES element. (D). porcine reschovirus serotype 1 (PTV-1) contains type IV IRES element.