There is still some controversy about the functions of the various subunits, which have been reviewed byLiu et al. previously (17). Briefly, PB1 contains conserved andwell-characterized RNA-dependent RNA polymerase motifs (3),while PB2 is required for transcription (16) and methylated capbinding (7, 9). PA has been implicated in a diverse range offunctions but has been confirmed to possess endonuclease activity