For adoptive T cell transfer studies of genetic inhibitors of HIV-1 infection, although in vitro transduction efficiencies resulting in more than 1 vector copy per cell have been obtained [14], after infusion into patients, the frequency of vector-containing CD4+ T cells in vivo has generally been in the range of 0.01% to 1% [14]–[18]. Here, CD4 is linked to HIV-1 infection.