Complex sentence: The overall incidence of myotubular myopathy and other centronuclear myopathies is unknown, but are far less common than more well-known muscle disorders like Duchenne's muscular dystrophy.
Split into simpler sentences: The overall incidence (frequency) of myotubular myopathy and other centronuclear myopathies is not known. They are understood to be far less common than more well-known muscle disorders like Duchenne's muscular dystrophy.