The Duchenne Muscular dystrophy (DMD) is the most frequent muscle disorder in childhood caused by mutations in the Xlinked dystrophin gene (about 65% deletions, about 7% duplications, about 26% point mutations and about 2% unknown mutations). The clinically milder Becker muscular dystrophy (BMD) is allelic to DMD. About 33% of all patients are due to de novo mutations and germ line mosaicism is frequently observed. While in earlier studies equal mutation rates in males and females had been reported, a breakdown by mutation types can better explain the sex ratio of mutations: Point mutations and duplications arise preferentially during spermatogenesis whereas deletions mostly arise in oogenesis. With current analytical methods, the underlying mutation can be identified in the great majority of cases and be used for carrier detection. However, in families with no mutation carrier available, the genetic model to be used for counselling of relatives can be quite complex.