The mdx mouse is a popular model for studying Duchenne muscular dystrophy (DMD).
The mdx mouse has a point mutation in its DMD gene, changing the amino acid coding for a glutamine to a threonine. This causes the muscle cells to produce a small, nonfunctional dystrophin protein. As a result, the mouse has a mild form of DMD where there increased muscle damage and weakness.
References
Mdx mouse Wikipedia(Text) CC BY-SA