In vertebrates all iron is taken up via the carrier protein transferrin. The carrier first binds its receptor and the receptor-ligand complex is then internalized via coated pits. The transferrin receptor is a transmembrane glycoprotein (apparent molecular weight (MW) 180,000) composed of two disulphide-bonded sub-units (each of apparent MW 90,000) It contains three N-linked glycan units and is post-translationally modified with both phosphate and fatty acyl groups. Here we have determined the nucleotide sequence of the coding region of the human transferrin receptor mRNA and from this deduced the amino acid sequence of the protein. The receptor does not contain an N-terminal signal peptide but there is a membrane-spanning segment 62 amino acids from the N-terminus. It therefore has a somewhat unusual configuration with a small N-terminal cytoplasmic domain and a C-terminal extracellular domain of 672 amino acids.