Ibalizumab, a humanized, anti-CD4 antibody binds the second domain of human CD4 and prevents HIV-1 infection in a non-competitive manner [11–13], whereas PRO140 competitively inhibits HIV-1 entry through binding the extracellular loop 2 region of the chemokine receptor 5 [14]. The gene discussed is CD4; the disease is HIV-1 infection.