Diagrams of Kell protein and of the recombinant expressed extracellular domain. The top diagram is of Kell protein showing the transmembrane region (TM), separating the short intracellular domain from a large extracellular segment. Also shown are the positions of cysteine residues (C), carbohydrate moieties (Y), and the zinc-binding enzymatic active site (HELLH). The bottom diagram depicts the expressed extracellular domain, devoid of intracellular and transmembrane domains and containing 6 histidines as a tag, plus an additional 4 amino acids (ADPG). The location of the introduced C72S mutation and a gp67 secretion signal that is cleaved after translation are also shown.