CD4 and infection: Interestingly, both stock virus strains grown in CEMx174 also retained certain key residues previously associated with CD4 dependent infection in non-human primates, including Met at position 165, Ile at 324, and (in 17E-Fred only) Lys at 573,the CD4 contact point of the conserved C3 loop GGDPE region [16-18].