Immunogen sequence: VCYDPKSSPG IWFEIHVGSK EGDLLNQTKV FPSGKDVVSL YFDVCQIVSM GSLFPVIFSS MEYYSSCHKN RYAHPACSTD SPVTTCWDCT TWSTNQQSLG PIMLTKIPLE PDCKTSTCNS VNLTILEPDQ PIWTTGLKAP L. Retroviral envelope proteins mediate receptor recognition and membrane fusion