Cardiac Troponin I (cTnI) is a protein found in the heart muscle cells. It is a subunit of the troponin complex, which plays a crucial role in the regulation of muscle contraction. The troponin complex is composed of three subunits: Troponin T (TnT), Troponin I (TnI), and Troponin C (TnC). When there is damage to the heart muscle, cTnI is released into the bloodstream, and its levels can be measured to diagnose a heart attack or other cardiac injuries.