CD71, a type II transmembrane glycoprotein expressed on the cell surface as a homodimer, is the transferrin receptor. It is essential for the growth of normal and neoplastic cells. It is expressed at low levels on resting B and T lymphocytes but is upregulated during responses to antigens and mitogens presumably reflecting the iron dependence of proliferation. Its expression declines with maturation and differentiation. CD71 plays a critical role in cell proliferation by controlling the supply of iron which is essential for many metabolic pathways through the binding and endocytosis of transferrin, the major iron-carrying protein. It may also be involved in signal transduction via its association with the T cell receptor ? chain. The monoclonal antibody RI7217 inhibits cell proliferation in vitro.