CD4 and AIDS: To date a number of different assays have been used (summarized in Polonis et al. and Ochsenbauer & Kappes, in Current Opinions in HIV and AIDS, Vol 4, No 5, 2009, Montefiori and Mascola, editors); most share the same characteristics including a CD4+/CCR5+ target cell, HIV-1 infectious molecular clones, pseudoviruses or virus-like particles as the challenge, and a marker of infection such as p24 Gag or luciferase.