A clue to the basis of allelic enhancement: occurrence of the Ax subgroup in the offspring of blood group O parents.
Olsson ML, Michalewska B, Hellberg A, Walaszczyk A, Chester MA. A clue to the basis of allelic enhancement: occurrence of the Ax subgroup in the offspring of blood group O parents. Transfus Med. 2005 Oct; 15(5):435-42.