xk (protein) xk (protein) xk (also known as kell blood group precursor) is a protein found on human red blood cells and other tissues which is responsible for the kx antigen which helps determine a person's blood type. xk (protein)identifierssymbolxkalt. symbolsxkr1, kx, x1kncbi gene7504hgnc12811omim314850pdbbae48708refseqnm_021083uniprotp51811other datalocuschr. x p21.2-p21.1search forstructuresswiss-modeldomainsinterpro clinical significance the kx antigen plays a role in matching blood for blood transfusions. mutation of xk protein may lead to mcleod syndrome, a multi-system disorder characterized by hemolytic anemia, myopathy, acanthocytosis, and chorea. xk is located on the x chromosome (cytogenetic band xp21.1) and absence of the xk protein is an x-linked disease. function xk is a membrane transport protein of unknown action.