No recombination was detected between two X-linked loci, RNR (Xp) and GPD (Xq), among 69 backcross progeny of two distantly related tammar wallaby (Macropus eugenii) subspecies. RNR loci are not dosage compensated, whereas the GPD locus is subject to the paternal X chromosome inactivation system that characterises female marsupials. The border of the region controlled by the marsupial X chromosome inactivation system has therefore been shown to lie between RNR and GPD.