Myofibrilar myopathy (MFM) is a neuromuscular disease characterized by slowly progressive muscle weakness that can involve both proximal muscles (such as hips and shoulders) and distal muscles (those further away from the trunk). Some affected individuals also experience muscle stiffness, aching, or cramps. Other symptoms that can be associated with MFM include pain and tingling in the limbs (peripheral neuropathy) or an enlarged and weakened heart (cardiomyopathy). Most people with MFM begin to develop muscle weakness in mid-adulthood, but features of the disease can appear anytime between infancy and late adulthood. MFM is caused by a genetic change (change) in any of several genes, including DES, CRYAB, MYOT, LDB3, FLNC, BAG3, FHL1, TTN, and DNAJB6. The signs and symptoms of MFM can vary depending on the genetic cause. For some people, the exact genetic cause may be unknown. The mode of inheritance of the disease depends on exactly which gene is changed. MFM can be diagnosed with a muscle biopsy or other studies of muscle function. The diagnosis can be confirmed with genetic testing.