The OTC gene coding for ornithine transcarbamylase is sex linked and subject to
Xinactivation in humans and mice. We have used a rat cDNA probe to localize OTC by in situhybridization in marsupials and monotremes. The gene maps to an autosomal site in two distantly related marsupial species and in one monotreme (the platypus); the first demonstration that a gene X-linked in one mammalian species may be autosomal in another. Since the conservation of the mammalian Xis thought to be a consequence of its isolation by the inactivation mechanism, we propose that an autosomal or pseudoautosomal segment containing OTC has been recruited into the inactivated region of the Xrather recently in eutherian evolution while it remained autosomal, or was translocated to an autosome, in metatherian and prototherian mammals.