We investigated structural and functional aspects of the first mutation in TNNC1, coding for the calcium-binding subunit (cTnC) of cardiac tropo-nin, which was detected in a patient with hypertrophic cardiomyopathy [Hoffmann B, Schmidt-Traub H, Perrot A, Osterziel KJ & Gessner R (2001) Hum Mut17, 524]. This mutation leads to a leucine–glutamine exchange at position 29 in the nonfunctional calcium-binding site of cTnC.