نبذه عن الكتاب: moving toward a molecular definition of these disorders, in the clinical context it is generally of limited value in only a small number of defined situations. Even when a disease gene has been identified, the types of mutation(s) encountered in a specific disease influence the practical availability of genetic testing (Table 2). For example, in myotonic dystrophy a single type of mutation (triplet-repeat expansion) is present in most affected individuals. Most patients with facioscapulohumeral MD can be shown to carry a DNA deletion, variable in size, but detectable using a single DNA probe (see Chap. 18). In both of these disorders, therefore, a relatively straightforward analysis can be employed to identify the majority of affected individuals, although in neither case is the molecular pathology of the disease understood. In the dystrophinopathies, the predominance of a particular type of mutation (intragenic deletion) simplifies the analysis in the majority of families (see Chap. 6). The continued need to apply a range of different tests, to provide as complete an answer as possible to the questions of carrier testing and prenatal diagnosis in these conditions means that, although such testing is widely available, it may remain complex (Chaps. 19 to 23). In other disorders, e.g., the limb-girdle muscular dystrophies (4), a range of point mutations have been described