as a cytological evidence, in first, mammalian x chromosomes in various species, including human and mouse, have nearly the same size, with the content of about 5% of the genome. second, for individual gene loci, a number of x-linked genes are common through mammalian species. examples are found in glucose-6-phosphate dehydrogenase (g6pd), a gene for polypeptide of antihemophilic globulin (ahg or factor viii) in hemophilia a and b, and plasma thromboplastin component gene (ptc or factor ix). moreover, no instances were found where an x-linked gene in one species was located on an autosome in the other species.