While the highly progressive Duchenne type of X-linked muscular dystrophy is characterised by genetic defects that result in the almost complete loss of the Dp427-M isoform of dystrophin in contractile tissues [30, 133], late-onset and less progressive Becker muscular dystrophy shows only reduced density and/or size of the dystrophin protein [170]. This evidence concerns the gene DMD and Becker muscular dystrophy.