gp120 Fragment 421-438; LYS-GLN-PHE-ILE-ASN-MET-TRP-GLN-GLU-VAL-GLY-LYS-ALA-MET-TYR-ALA-PRO-PRO; HIV ENVELOPE PROTEIN (GP120) FRAGMENT 421-438; HIV (GP120) FRAGMENT (421-438); H-LYS-GLN-PHE-ILE-ASN-MET-TRP-GLN-GLU-VAL-GLY-LYS-ALA-MET-TYR-ALA-PRO-PRO-OH; gp120 fragment 421-438