The methylation of the 5' terminal guanosine residue of the cap structure of Semliki Forest virus (SFV) mRNAs has been shown to occur in vitro concomitantly with their synthesis (R. K. Cross and P. J. Gomatos, Virology, 114, 542-554, 1981). The enzyme responsible for this methylation, a guanine-7-methyltransferase, is associated with the SFV replication complex