Duchenne/Becker muscular dystrophies (DMD/BMD) are the most common inherited muscular disease and caused by mutations in the dystrophin gene. A half to two-thirds of DMD and BMD patients carry deletions (usually of several kilobases of genomic DNA). The clinical progression in DMD and BMD patients with deletions can be predicted in 92% of cases based on whether the deletion maintains or disrupts the translational reading frame (frame-shift hypothesis). However, some exceptional cases have been reported in which some posttranscriptional modifications were suggested, such as alternative splicing and reinitiation of translation. Splicing mutation is one kind of mutations of dystrophin gene, and usually induced by a small mutation of exon-intron boundary sequence. However, intraexonal small mutation also induces exon skipping, due to disruption of an exon recognition sequence, which is an intraexonal sequence and necessary for splicing of the upstream intron. Carrier diagnosis is one of the important clinical application of genetic diagnosis. In the case of DMD/BMD with deletions of the dystrophin gene, carrier diagnosis is difficult because of the existence of normal X chromosome. In these cases a linkage analysis is useful, and in some cases non-carriers can be directly diagnosed on the basis of microsattelite polymorphism detected in deleted region of patient. For the molecular diagnosis of DMD/BMD it is important to analyze not only at the genomic DNA level, but also at the mRNA, protein, and clinical levels. And the relationship between the molecular abnormality and clinical phenotype should be examined, especially extramuscular symptoms such as heart failure and mental retardation.