The invention relates to the treatment, diagnosis, and prognosis of a muscular dystrophy or myopathy. The present inventors have found that the quantity of mu-crystallin is increased in a muscular dystrophy. In particular, the inventors have found that mu-crystallin is increased in facioscapulohumeral muscular dystrophy (FSHD). Based on the inventors' findings, the invention provides a novel means for the treatment, diagnosis, and prognosis of a muscular dystrophy or myopathy.