CD4 and infection: A similar increase in infection was observed for E153G, particularly at low CD4 expression (compare blue and green bars in Figure 3C to A), whereas the K421D mutant showed inefficient entry at low CCR5 levels regardless of how much CD4 was present (Figure 3D, note the low infectivity at 0 and 0.25 μM PonA (<20% of maximum) even when CD4 was maximally induced).