DMD and Becker muscular dystrophy: The differences in the two types of diseases are attributed to a translational reading frame rule (9); mutations shifting the translational reading frame of the mRNA usually result in the more severe DMD phenotype because of the creation of premature stop codons that totally prohibit dystrophin protein expression, whereas mutations that maintain the original mRNA reading frame cause the milder BMD phenotype because a mutated but still functional dystrophin protein can be expressed from the mRNA.