Murine X-linked genes corresponding to the human Fragile X (FMR1) and Hunter syndrome (IDS) loci have been mapped in an interspecific backcross between B6CBA-Aw-J/A-Bpa and Mus spretus using human cDNA clones. Pedigree analysis of recombinants from a total of 248 backcross progeny favors a gene order of (Cf-9, Mcf-2)-(Fmr-1)-Ids-Gabra3-Rsvp. Gene order is conserved between the species, although no fragile site has been detected in the mouse in this region of the murine X chromosome.