Cells infected with these viruses were notably less fluorescent than RA59-gfp, suggesting that the N-terminal addition to the EGFP protein may interfere with its function. Alternatively there may be less transcription or translation due to differences near the intergenic sequences (5, 20). RA59-gfp/gp33 (log10 50% lethal dose [LD50] = 5.6) displayed a similar level of virulence to RA59-gfp (log10 LD50 = 5.5). As previously published (5), RA59-gfp is somewhat attenuated relative to wild-type recombinant RA59 (log10 LD50 = 4); thus, both EGFP-expressing viruses display LD50 values approximately 10-fold higher than those displayed by wild-type RA59.