Virions produced in the presence of L4 and K2 LF Nefs (CD3 retention and increased infectivity) required more antibody to neutralize infection by 50% and thus displayed significantly higher 50% inhibitory concentration IC50 values when compared to virions encoding the L4 and K3 IL Nef proteins (Fig. 3 J and K and SI Appendix, Fig. S5 F and G). This evidence concerns the gene LTF and infection.