Cardiac troponin I, often denoted as cTnI, is presented in cardiac muscle tissue by a single isoform with molecular weight 23876 Da and it consists of 209 amino acid residues. The theoretical pI of cTnI is 9.87. cTnI differs from other troponins due to its N-terminal extension of 26 amino acids. This extension contains two serines, residues 23 and 24, which are phosphorylated by protein kinase A in response to beta-adrenergic stimulation and important in increasing the inotropic response.[1] Phosphorylation of cTnI changes the conformation of the protein and modifies its interaction with other troponins as well as the interaction with anti-TnI antibodies. These changes alter the myofilament response to calcium, and are of interest in targeting heart failure. Multiple reaction monitoring of human cTnI has revealed that there are 14 phosphorylation sites and the pattern of phosphorylation observed these sites is changed in response to disease.[2] cTnI has been shown to be phosphorylated by protein kinase A, protein kinase C, protein kinase G, and p21-activated kinase 3.[3] A significant part of cTnI released into the patient’s blood stream is phosphorylated.[4] For more than 15 years cTnI has been known as a reliable marker of cardiac muscle tissue injury.