Creatine kinase, muscle also known as MCK is a creatine kinase. The protein is a cytoplasmic enzyme involved in cellular energy homeostasis. The 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 MB-CK consisting of one muscle (M-CK) and one brain-type (B-CK) subunit is expressed. The latter may be an important serum marker for myocardial infarction, if released from damaged myocardial cells into the blood where it can be detected by clinical chemistry.