CD4 and infection: As CCR5+CD4+ T cells would be a major producer of HIV-1 on the basis of their higher activation level, our previous finding would provide an insight into a mechanism by which R5 HIV-1 is dominantly isolated during the early stage of infection in patients [90,91,92], despite the fact that newly infected individuals have usually been exposed to a mixture of R5 and X4 viruses [93,94,95,96].