McLeod syndrome is an X-linked multisystem disorder characterized by 
abnormalities in the neuromuscular and hematopoietic systems. We have assembled 
a cosmid contig of 360 kb that encompasses the McLeod gene locus. A 50 kb 
deletion was detected by screening DNA from patients with radiolabeled whole 
cosmids, and two transcription units were identified within this deletion. The 
mRNA expression pattern of one of them, designated as XK, correlates closely to 
the McLeod phenotype. XK encodes a novel protein with structural characteristics 
of prokaryotic and eukaryotic membrane transport proteins. Nucleotide sequence 
analysis of XK from two unrelated McLeod patients has identified point mutations 
at conserved splice donor and acceptor sites. These findings provide direct 
evidence that XK is responsible for McLeod syndrome.