Comparative mapping of the X chromosome in eutherian mammals has revealed distinct regions of conservation as well as evolutionary rearrangements between human and mouse. Recently, we and others mapped the murine homologue of CLCN4 (Chloride channel 4) to band F4 of the X chromosome in Mus spretus but to chromosome 7 in laboratory strains. We now report the mapping of the murine homologues of APXL (Apical protein Xenopus laevis-like) and OA1 (Ocular albinism type I), two genes that are located on the human X chromosome at band p22. 3 and in close proximity to CLCN4. Interestingly, Oa1 and Apxl map to bands F2-F3 in both M. spretus and the laboratory strain C57BL/6J, defining a new rearrangement between human and mouse X chromosomes.