Although HIV-1 infects and depletes CD4+ T lymphocytes to cause acquired immunodeficiency syndrome in vivo, infection of primary CD4+ T lymphocytes by cell-free virus was inefficient in vitro after two days of continuous culture (Fig 1C and S1A Fig) using an inoculum comparable to the amount of virus present in MDM-T lymphocyte co-cultures. This evidence concerns the gene CD4 and AIDS.