Troponin I (TnI) is a protein that plays a crucial role in muscle contraction, particularly in cardiac muscle (the heart) and skeletal muscle. It is one of the three subunits of the troponin complex, along with troponin T (TnT) and troponin C (TnC). The troponin complex is found in striated muscle, where it regulates the interaction between actin and myosin filaments, leading to muscle contraction.