Rps4 maps near the inactivation center on the mouse X chromosome

RPS4Y, a Y-linked gene in humans, appears to encode an isoform of ribosomal protein S4. A homologous locus on the human X chromosome, RPS4X, lies close to the X-inactivation center but fails to undergo X-inactivation. We have isolated a genomic clone from the mouse Rps4 locus, the homolog of human RPS4X. We derived an intron probe that hybridizes to the functional Rps4 locus but does not cross-hybridize to related sequences elsewhere in the mouse genome. Genetic mapping utilizing interspecific mouse backcrosses and the intronspecific probe demonstrates that Rps4 maps close to the Phka locus on the mouse X chromosome and in the vicinity of the X-inactivation center. The gene order Ccg-1-Rps4 Phka-Xist-Pgk-1 is conserved between mouse and human.