A DNA marker C7, localised Xp21.1-Xp21.3, has been studied in kindreds segregating for Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD). In DMD families four crossovers were observed in 38 informative meioses between C7 and the DMD locus (theta = 0.12, z max = +2.72). In BMD families no recombinants were observed in the 16 informative meioses studied. These data are consistent with the localisation of the mutations in these disorders being in the same region of Xp21. Studies in families also segregating for the DNA marker 754 support the previously reported physical order of these loci as X centromere-754-DMD-BMD-C7-X telomere. A recombination fraction of 0.11 (z max = +5.58) was found between DMD-754 by combining our previously published data with the data presented here. C7 and 754 thus provide good bridging markers for the diagnosis of DMD and BMD.