X-Kell Blood Group Precursor Protein (XK)

XK 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. The Kx antigen plays a role in matching blood for blood transfusions. Absence of XK protein leads to McLeod syndrome, a multi-system disorder characterized by hemolytic anemia, myopathy, acanthocytosis, and chorea.

XK is located on the X chromosome and absence of the XK protein is an X-linked disease. XK is a membrane transport protein of unknown action.The XK locus is inactivated by lyonization. The precursor missing in McLeod's red cells is called Kx. The X-linked locus determining this substance is called XK.