HIV-1 infection decreases the levels of helper T-cells through a number of mechanisms, such as apoptosis of uninfected bystander cells (Garg et al., 2012), direct viral killing of infected cells, and killing of infected CD4+ T cells by CD8+ cytotoxic lymphocytes that recognize infected cells (Kumar et al., 2012), and causes acquired immunodeficiency syndrome (AIDS). This evidence concerns the gene CD4 and AIDS.