Creatine phosphokinase (CPK), also known as creatine kinase (CK), is an enzyme derived from rabbit muscle. This Type I preparation, with a minimum activity of 150 U/mg, is used as a reagent in biochemical research to study muscle energy metabolism and ATP-creatine phosphate shuttling. It is also employed in in vitro diagnostic assays for the quantitative determination of CK activity in serum, aiding in the diagnosis of myocardial infarction, muscular dystrophy, and other conditions involving muscle damage.