Although less ubiquitous, helicases are also found in other types of viruses (see some examples listed in Table 1 ). Direct mutagenesis studies have confirmed that a helicase function is biologically required for the replication of many viruses including vaccinia virus (19) , poliovirus (20) , alphaviruses (21) , brome mosaic virus (22) , nidoviruses (23, 24) and flaviviruses (25) (26) (27) .