CD4 (cluster of differentiation 4), also known as L3T4, is a 55 kDa glycoprotein . It is a member of the Ig superfamily and is primarily expressed on T helper cells, on regulatory T cells, at lower levels on subpopulations of NKT cells, some populations of dendritic cells, and on thymocytes (CD4+CD8+ and CD4+CD8‚¬€œ thymocytes). CD4 acts as a coreceptor with the TCR during T cell activation and thymic differentiation by binding MHC class II and associating with the protein tyrosine kinase, lck. This antibody is widely used to detect mouse CD4 T cells and to deplete CD4 T cells in vivo.