Two dissimilar brothers with Becker's dystrophy have an identical genetic defect.

Becker's muscular dystrophy is phenotypically heterogeneous, but the clinical expression is usually similar in patients within the same family. We report here 2 brothers affected with Becker's muscular dystrophy in whom the disease followed completely different courses. The disease started in both patients before their teens. However, the oldest sibling died at 37, following many years of severe disability, whereas the other sibling, now 26, has normal muscle strength. In addition, since the age of 13, the younger brother has had epilepsy and has been treated with phenytoin combined with other antiepileptic drugs. Analysis of the DNA from each of the 2 brothers revealed a similar deletion at the 5' end of the dystrophin gene. The different clinical courses despite the similar mutational event suggest that intrinsic muscle factors due to modified genes or environmental phenomena such as prolonged treatment with phenytoin or other antiepileptic agents may have influenced the clinical course.