An individual with McLeod syndrome and the Kell blood group antigen K(K1).

McLeod syndrome is an X-linked condition in which individuals of McLeod blood group phenotype have weak Kell antigens, acanthocytic red cells, and a muscular disorder. We now report a family in which two brothers have McLeod syndrome. One is K:-1, while the other is the first known K:1 person with McLeod syndrome. The K1 gene in the latter is expressed weakly and was inherited from the father, in whom it is expressed normally. The brothers have the same clinical and laboratory manifestations of McLeod syndrome but have different Kell genes. Therefore, the Kell gene is unlikely to have any positive input into development of McLeod syndrome; its role is one of passive involvement in which its expression is modified.