The molecular basis for Duchenne and Becker muscular dystrophies is reviewed from the Department of Pediatric Neurology, Floating Hospital for Infants and Children, New England Medical Center Hospitals and Tufts University, Boston, Mass. Until recently the diagnosis of DMD or BMD depended on clinical signs and symptoms, serum creatine kinase, EMG and muscle biopsy. The isolation of the gene defective in DMD and BMD and the identification of dystrophin have revolutionized the diagnostic issues. The mutated gene causing Duchenne and its allelic milder Becker phenotype has been assigned to band P21 of the short arm of the X chromosome (Xp21). Dystrophin has been characterized by DNA sequencing and by immunologic studies. When the family history is negative for DMD and BMD the Western Blot assay of protein derived from a specimen can confirm the clinical diagnosis of DMD or BMD and can be used to predict the severity of the disease. If the dystrophin assay result is abnormal DNA analysis should be performed. Detection of a dystrophin gene deletion will facilitate carrier detection and prenatal diagnosis in the proband's family. If no deletion or duplication is found, linkage analysis may be attempted for prenatal diagnosis and carrier detection. Peripheral blood DNA may be used for Southern Blot testing if a muscle biopsy specimen is not available and may confirm the diagnosis in more than 65% of the cases. In typical cases of DMD or BMD with a family history of X linked muscular dystrophy, linkage analysis is unnecessary if the clinical diagnosis has been confirmed in an affected family member by analysis of dystrophin or DNA or both. The less invasive polymerase chain reaction test may be used if the diagnosis has not been confirmed by dystrophin or DNA analysis in other family members. Muscle biopsy for dystrophin analysis will be required in families without a clear cut X linked pattern of inheritance or in families with both male and female siblings affected, suggesting an autosomal recessive form of MD. 
COMMENT: Gross abnormalities of the dystrophin gene may still result in a partially functional dystrophin protein and a relatively mi Id clinical progression, compatible with a diagnosis of Becker muscular dystrophy. Angelini C et al.  describe a patient with a duplication of more than 400,000 bp of the dystrophin gene, the largest characterized to date. The propositus a 13 year old boy presented at age 4 with myalgia and cramps after exercise or running in the cold. The CK ranged from 1400 to 8630 U/l, the EMG showed small polyphasic motor units, and the muscle biopsy revealed a mild myopathic picture with scattered atrophic and hypertrophic muscle fibers, a few degenerating fibers and a mild inflammatory reaction. Electronmicroscopy showed hypertrophic ring fibers. Leg muscle ultrasound revealed scattered fibrosis. He was treated for five months with prednisone at 50 mg/d followed by two months at 50 mg on alternating days. The CK levels declined and the child had less muscle pain.