To improve the model and particularly to enable systemic HIV-1 infection after mucosal exposure, we are endeavoring to increase the capacity of mouse CD4+ T cells to support HIV-1 replication by identifying the human genes required to overcome other species-specific blocks to HIV-1 replication which could be expressed as transgenes in hCD4/R5/cT1 mice. The gene discussed is CD4; the disease is HIV-1 infection.