The surface glycoprotein Env is a primary determinant of cell tropism; in early infection the virus targets CD4+ helper T cells and macrophages primarily, while in later infection tropism extends to CD8+ T cells and B cells [11], [12] and it would appear that early and late isolates of virus may differ in the way they interact with the primary receptor CD134 and their propensity for CD134-independent infection [13], [14]. This evidence concerns the gene CD4 and infection.