The monoclonal antibody ER-MP21 recognizes CD71, the transferrin receptor 1. CD71 is a 200 kDa glycoprotein composed of two identical, disulfide-linked chains. Each chain is capable of binding transferrin, an iron-transport molecule. Binding of transferrin to its receptor followed by endocytosis of the receptor-ligand complex is a major cellular iron-uptake mechanism. Iron uptake by the proliferating cell is essential for the iron-containing enzyme ribonucleotide reductase involved with DNA synthesis. CD71 is not only expressed by cycling cells, but also by cells that require iron for other iron-dependent proteins, such as the enzyme peroxidase in monocytes. Rapidly proliferating cells, as in malignancy, generally express CD71 abundantly. Furthermore, CD71 has been described as a marker of immature, proliferating T cells.The monoclonal antibody ER-MP21 inhibits proliferation and differentiation during early T cell development. ER-MP21 recognizes murine CD71, but does not compete with transferrin binding.