Sequence homology of HIV-1 gp120 with rabies virus glycoprotein and snake venom neurotoxins. Boldface letters indicate amino acid identities between gp120 and the rabies virus glycoprotein, italic letters indicate identities between gp120 and snake neurotoxins, italic bold letters are used for amino acids that are identical in gp120, rabies virus glycoprotein, and snake neurotoxins.