Creatine kinase (CK) is a key metabolic enzyme. It is a dimer of two subunits, each with molecular weights of approximately 43 kDa. Two different subunits are expressed, M and B. The CK holoenzyme exists as MM and BB homodimers and an MB heterodimer. The MM and BB isoforms are expressed primarily in skeletal muscle and brain respectively while both MB and MM are expressed in heart. CK-MM was purified from skeletal muscle using affinity chromatography. It has not been evaluated for enzyme activity.