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Mexiletine. A drug used to treat myotonia (delayed muscle relaxation after contraction) in muscle diseases such as myotonic dystrophy and myotonia congenital (brand name is Mexitil).

Modafinil. A drug is used to treat excessive daytime sleepiness (brand name is Provigil).

Motility. The power of spontaneous movement.

Multisystemic disorder. A disease that can affect many different organs and systems in the body.

Mutation. As used on this website, a change in the normal chemistry of a gene.

Myotonia. The inability of contracted muscles to relax on command; or, a special kind of muscle stiffness.

Myopathy. Muscle weakness.