CD4 and infection: Infectivity of the 4059 T-tropic Env pseudotyped virus was approximately 1,000-fold over background but was 10-fold lower than that of the M-tropic Env viruses, indicative of an intermediate phenotype that we have found previously in people with compartmentalized CNS infection within the first 2 years of infection (74) and that can be found in the blood in some people with low CD4 T cell counts (Bednar M, Hauser B, Zhou S, Cohen M, Joseph SB, Swanstrom R, in preparation).