Transmission of the X4 Phenotype of HIV-1: Is There Evidence Against the “Random Transmission” Hypothesis?

same biological characteristics of HIV-1 isolates. The Dutch group also developed the MT-2 cell line assay as a convenient method to distinguish between the 2 biological phenotypes of HIV-1 [ 3 ]. Subsequent studies showed that MT-2–positive isolates occasionally are observed in patients with primary HIV infection, even though most asymptomatic patients have MT-2–negative isolates [ 4 – 7 ]. Furthermore, a switch from MT-2– negative to MT-2–positive isolates occurs in approximately 50% of patients during progression toward end-stage AIDS and is associated with an increased rate of CD4 cell decline [ 6 , 7 ]. These findings were interpreted to suggest that MT-2–positive isolates are more virulent in vivo, and their appearance is both a cause and