Résumé
DNA sequence polymorphisms (RFLPs) have been widely used as genetic markers for identification of the X chromosome that carries the mutation for Duchenne muscular dystropby (DMD) in affected families, but serious limitations and pitfalls are associated with this approach [Darras et al., 1987]. The complementary DNA (cDNA) of the DMD gene has recently been isolated and shown to detect partial gene deletions in a large proportion of patients [Koenig et al., 1987]. Two prenatal studies are presented to illustrate how the unambiguous identification of deletion mutations by cDNA probes permits direct DNA‐based diagnoses with high accuracy and in otherwise uninformative families. In a single proband family, DNA marker analysds had determined that the Xp21 chromosomal region present in the affected male was also carried by a male fetus in a oubsequent pregnancy. Analysis of this family's DNA with probes covering the entire 14 kb cDNA revealed a small deletion in the affected male that was not present in the fetus nor in the mother. In the second family the fetus was a female deletion carrier identified by comparing intensities of restriction fragments. Since 1/3 of all DMD patients are thought to result from new mutations and most families have only single affected males, the cloned cDNA probes now available are likely to revolutionize DNA‐based diagnostic studies in this disorder. More reliable, more rapid and less expensive than linkage studies with DNA polymorphisms, this methid will be informative in the more than 50% of DMD/BMD cases that have deletion mutations.