CD4 and infection: Rather, the tight association between CD4 binding and infection of CD4low cells of the evolving R5 viruses in both BR24 and CA28, and the finding that primary macrophages are the principle virus-producing cells at end-stage disease in these two macaques with coreceptor switch suggest that adoption of an “open” Env is in response to the need to use low levels of CD4 receptor more efficiently.