The 5' nontranslated RNAs of hepatitis C virus (HCV) and several other members of the Flaviviridae contain highly structured segments which form internal ribosome entry sites (IRESs). These cia-active RNA elements direct the cap-independent initiation of translation of the viral polyprotein in association with trans-acting cellular and possibly viral proteins, and thus they play a key role in the replication of the virus. The structure of the HCV IRES does not resemble that of any picornaviral IRES, and its function is uniquely dependent upon RNA sequence extending 3' of the site of translation initiation as well as structure surrounding the initiator AUG. (C) 1997 Academic Press.