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[Display omitted] • We here present the crystal structures of 7 wild type fibronectin type III domains from the massive sarcomeric protein titin. • The crystal structures of two of these domains containing missense variants found in patients with the heart disease hypertrophic cardiomyopathy (HCM) were also determined and showed little difference to the wild type domains. • Five missense variants found in patients with HCM were shown to destabilise their domains by 7.4–13.0 °C. • Analysis of crystal contacts revealed no evidence for physiological domain dimerization. Titin is the largest protein found in nature and spans half a sarcomere in vertebrate striated muscle. The protein has multiple functions, including in the organisation of the thick filament and acting as a molecular spring during the muscle contraction cycle. Missense variants in titin have been linked to both cardiac and skeletal myopathies. Titin is primarily composed of tandem repeats of immunoglobulin and fibronectin type III (Fn3) domains in a variety of repeat patterns; however, the vast majority of these domains have not had their high-resolution structure determined experimentally. Here, we present the crystal structures of seven wild type titin Fn3 domains and two harbouring rare missense variants reported in hypertrophic cardiomyopathy (HCM) patients. All domains present the typical Fn3 fold, with the domains harbouring variants reported in HCM patients retaining the wild-type conformation. The effect on domain folding and stability were assessed for five rare missense variants found in HCM patients: four caused thermal destabilization of between 7 and 13 °C and one prevented the folding of its domain. The structures also allowed us to locate the positions of residues whose mutations have been linked to congenital myopathies and rationalise how they convey their deleterious effects. We find no evidence of physiological homodimer formation, excluding one hypothesised mechanism as to how titin variants could exert pathological effects. [ABSTRACT FROM AUTHOR]