AbstractObjectiveThe giant protein titin is essential for striated muscle development, structure, and elasticity. All titin mutations reported to date cause late‐onset, dominant disorders involving either skeletal muscle or the heart. Our aim was to delineate the phenotype and determine the genetic defects in two consanguineous families with an early‐onset, recessive muscle and cardiac disorder.MethodsClinical and myopathological reevaluation of the five affected children, positional cloning, immunofluorescence, and Western blot studies were performed.ResultsAll children presented with congenital muscle weakness and childhood‐onset fatal dilated cardiomyopathy. Skeletal muscle biopsies showed minicores, centrally located nuclei, and/or dystrophic lesions. In each family, we identified a homozygous titin deletion in exons encoding the C‐terminal M‐line region. Both deletions cause a frameshift downstream of the titin kinase domain and protein truncation. Immunofluorescence confirmed that truncated titins lacking the C‐terminal end were incorporated into sarcomeres. Calpain 3 was secondarily depleted.InterpretationM‐line titin homozygous truncations cause the first congenital and purely recessive titinopathy, and the first to involve both cardiac and skeletal muscle. These results expand the spectrum of early‐onset myopathies and suggest that titin segments downstream of the kinase domain are dispensable for skeletal and cardiac muscle development, but are crucial for maintaining sarcomere integrity. Ann Neurol 2007;61:340–351