Calnexin is a molecular chaperone that facilitates folding of glycoproteins in the endoplasmic reticulum (ER). The cloned lumenal domain of canine calnexin, cnx Delta TMC, retains its biological activity without the transmembrane and cytosolic region. For the purpose of structure determination we generated a crystallizable core by mild proteolysis and identified its termini by N-terminal sequencing and molecular mass determination. A truncated gene was cloned accordingly. Its product, cnx Delta N25C15, was purified to apparent homogeneity and crystallized. This truncated variant remains biologically active as shown by its binding to monoglucosylated oligosaccharides and functional interaction with ERp57. A heavy atom derivative was identified.