To help optimize the receptor and co-receptor requirements for infection, different CD4 and CXCR4 combinations (Figure 1B) were tested for their capacities to infect cells expressing Env-753Stop (Figure 5A) and Env-WT (Figure 5B), and normalized to results of the CD4-GPI (gpi) plus CXCR4-318 (318). Here, CXCR4 is linked to infection.