How does the iron (Fe) binding protein, transferrin (Tf), bind to the transferrin receptor 1 (TfR1) to donate Fe to cells? In this issue of Cell, Cheng et al., describe the molecular structure of the human TfR1-Tf complex, This atomic model shows that Tf binds laterally to the TfR1 dimer and extends into the gap between the bottom of the receptor ectodomain and the membrane.