CD4 is a cell-surface glycoprotein found on the mature helper T cells and immature thymocytes, as well as on monocytes and macrophages. (Some cytotoxic T cells have CD4 protein as well.) Normally, about 65 % of T cells in the blood are CD4+ (have CD4 protein protruding from their membrane). A mature T cell with either have CD4 or CD8, but not both. During one stage of development T cells develop CD4 and CD8 receptors, but they eventually are differentiated in the thymus and become more specialized. Alternative Names: HLA-2 protein, soluble CD4 protein, CD4mut protein, CD 4, T cell antigen T4/LEU3 protein, Ly-4 protein, L3 / T4 protein, CD4, CD-4, CD 4 protein, CD-4 protein, sCD4 protein, T4 protein, gp55 protein