Among the movement disorders associated with acanthocytosis, McLeod syndrome 
(McKusick 314850) is the one that is best characterized on the molecular level. 
Its defining feature is low reactivity of Kell erythrocyte antigens. This is due 
to absence of membrane protein KX that forms a complex with the Kell protein. KX 
is coded for by the XK gene on the X-chromosome. We present six males (aged 29 
to 60 years), with proven XK mutations, to discuss the chorea associated with 
McLeod syndrome. The movement disorder commonly develops in the fifth decade and 
is progressive. It affects the limbs, the trunk and the face. In addition to 
facial grimacing, involuntary vocalization can be present. In early stages there 
may only be some restlessness or slight involuntary distal movements of ankles 
and fingers. Lip-biting and facial tics seem more common in autosomal recessive 
choreoacanthocytosis linked to chromosome 9. This, together with the absence of 
dysphagia in McLeod syndrome, may help in differential diagnosis. Recent 
findings suggest a role for the endothelin system of the striatum in the 
pathogenesis of McLeod syndrome.