The severe Duchenne and milder Becker muscular dystrophy are both caused by mutations in the DMD gene. This gene codes for dystrophin, a protein important for maintaining the stability of muscle‐fiber membranes. In 1988, Monaco and colleagues postulated an explanation for the phenotypic difference between Duchenne and Becker patients in the reading‐frame rule: In Duchenne patients, mutations induce a shift in the reading frame leading to prematurely truncated, dysfunctional dystrophins. In Becker patients, in‐frame mutations allow the synthesis of internally deleted, but largely functional dystrophins. Currently, over 4700 mutations have been reported in the Leiden DMD mutation database, of which 91% are in agreement with this rule. In this study we provide an update of the mutational variability in the DMD gene, particularly focusing on genotype–phenotype correlations and mutations that appear to be exceptions to the reading‐frame rule. Muscle Nerve, 2006