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.
^Arnaud L, Salachas F, Lucien N, et al. (March 2009). "Identification and characterization of a novel XK splice site mutation in a patient with McLeod syndrome". Transfusion. 49 (3): 479–84. doi:10.1111/j.1537-2995.2008.02003.x. PMID19040496. S2CID27198922.
^Malandrini, A; Fabrizi, GM; Truschi, F; Di Pietro, G; Moschini, F; Bartalucci, P; Berti, G; Salvadori, C; et al. (1994). "Atypical McLeod syndrome manifested as X-linked chorea-acanthocytosis, neuromyopathy and dilated cardiomyopathy: report of a family". Journal of the Neurological Sciences. 124 (1): 89–94. doi:10.1016/0022-510X(94)90016-7. PMID7931427. S2CID27859436.