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Structural view of the role of the hemochromatosis protein HFE, a class I major histocompatibility complex (MHC) homolog, in regulating iron uptake by cells. Foreground: two HFE molecules (red and orange ribbons) bound to a homodimeric transferrin receptor (blue ribbons) as seen in the 2.8 angstrom crystal structure of an HFE-transferrin receptor complex. Background: HFE competes with the serum iron carrier protein transferrin (green; based upon the structure of lactoferrin PDB code 1LFG) for binding to transferrin receptor.