Mondo Description A disorder of the musculoskeletal system caused by pathogenic variants in the TTN gene encoding the titin protein expressed in striated muscle. These variants are associated with a variety of overlapping congenital and adult-onset myopathies characterized by non-progressive or progressive neck, axial, and limb weakness, joint contractures, early-onset respiratory insufficiency, facial weakness, congenital cardiac anomalies and/or early-onset dilated cardiomyopathy. Histologic findings on skeletal muscle biopsy reveal a wide range of structural abnormalities and can include increased internalized and central nuclei, minicores, and dystrophic changes.