COL6A3 and Ullrich congenital muscular dystrophy: Ullrich congenital muscular dystrophy (UCMD), Bethlem myopathy (BM), and myosclerosis myopathy (MM) are diseases caused by mutations in each of the three genes (COL6A1, COL6A2, and COL6A3) encoding the extracellular matrix protein collagen VI.