The role of human CD4+ αβT cells in host defense has been delineated by the natural history of patients with two immunodeficiencies that affect the numbers of these cells in peripheral blood: acquired immune deficiency syndrome (AIDS), secondary to human immunodeficiency virus (HIV) infection (Dalgleish et al., 1984; Klatzmann et al., 1984; Maddon et al., 1986), and inherited MHC class II deficiency due to inborn errors of transcription factors that govern expression of these human histocompatibility leukocyte antigens (HLA) loci (Nekrep et al., 2003). The gene discussed is CD4; the disease is Immunodeficiency.