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 exist, M and B. The CK holoenzyme therefore 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. The relatively high expression levels of the MB isoform in heart explains its use as an established biomarker for heart disease. Likewise, CK-MM can be used as a specific biomarker for skeletal muscle injury.