HIV progresses to acquired immunodeficiency syndrome (AIDS) when a patient’s CD4+ cell count is below 200 cells/mm3 of blood or when a patient presents with any of a number of AIDS-defining illnesses (e.g. Kaposi sarcoma, HIV-related encephalopathy, Pneumocystis jirovecii) regardless of CD4+ count. The gene discussed is CD4; the disease is AIDS.