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.1, 4 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.4 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 ironcarrying protein. It may also be involved in signal transduction via its association with the T cell receptor zeta chain. The monoclonal antibody RI7217 inhibits cell proliferation in vitro.2