TNNI3 and restrictive cardiomyopathy: Mutations in the gene encoding the cardiac troponin I (TNNI3) account to around 3% of HCM and also have a heterogeneous phenotype.6 More than 55 mutations in TNNI3 have been reported to cause cardiomyopathy, mostly hypertrophic but also a minority that can cause a dilated or restrictive cardiomyopathy.9,10 Troponin is a protein complex made of troponin I, troponin C, and troponin T and is located within the thin filament of the sarcomere where it is responsible for binding calcium and switching contraction.