– Kell blood group glycoprotein (KEL) is a single-pass type II membrane protein which belongs to the peptidase M13 family. It is expressed in Expressed at high levels in erythrocytes and testis, and, at lower levels, in skeletal muscle, tonsils, lymph node, spleen and appendix. KEL has been shown zinc endopeptidase with endothelin-3-converting enzyme activity. It cleaves EDN1, EDN2 and EDN3, with a marked preference for EDN3. It links via a single disulfide bond to the XK membrane protein that carries the Kx antigen.