is a cytoplasmic enzyme involved in cellular energy homeostasis. The encoded protein reversibly catalyzes the transfer of "energy-rich" phosphate between ATP and creatine and between phospho-creatine and ADP. Its functional entity is a MM-CK homodimer in striated (sarcomeric) skeletal and cardiac muscle.In heart, in addition to the MM-CK homodimer also the heterodimer consisting of one muscle (MM-CK) and one brain-type (MB-CK) subunit is expressed. This latter is an important serum marker for myocardial infarction, for it is released from damaged myocardial cells into the blood where it can be detected by clinical chemistry. Since no other organ than cardiac muscle expresses significant levels of MB-CK, this clinical test is still fairly reliable, together with troponin-T, for diagnosing a myocardial infarction.