From the Erlwein letter: The technical abbreviations here are aa = amino acid, ORF = open reading frame. This pitch is not so much aimed at Lo/Alter as at the infectious agent itself. How dare it persist in this defective form? We now have a variety of helper viruses with the full Gag sequence available. We see considerable sequence variation in available data, suggesting variation is common, if not inherent. The explanation is already shaping up nicely. What is your problem? As umpire, I would definitely call this a ball.