Immunogen sequence: QLAENIASSL HVASCYVCGG MNMGDQWPWE ARELMPQDNF TLTASSLEPA PSSQSIWFLK TSIIGKFCIA RWGKAFTDPV GELTCLGQQY YNETLGKTLW RGKSNNSESP HPSPFSRFPS LNHSWYQLEA. Retroviral envelope proteins mediate receptor recognition and membrane fusion during early