In the mdx mouse models of DMD, the frequency of occurrence of these clusters varies between individual mdx mutant strains (Danko et al. 1992; Fritz et al. 1995), being highest in the originally described C5BL10 mdx strain where a point mutation introduces a stop codon into exon23, despite which, dystrophin positive ‘revertant’ muscle fibres are regularly seen in frozen sections. This evidence concerns the gene DMD and Duchenne muscular dystrophy.