In the absence of ART, HIV can establish a progressive infection of human immune cells leading to eventual loss of the CD4+ T cells [49] over the course of HIV disease progression (stage 1, CD4+ cell count is above 500 cells/μL; stage 2, CD4+ cell count is between 350 and 500 cells/μL; stage 3, CD4+ cell count is between 200 and 350 cells/μL; stage 4, CD4+ cell count is less than 200 cells/μL). The gene discussed is CD4; the disease is infection.