Creatine kinase (a.k.a. creatine phosphokinase or CPK) is a muscle enzyme which exists as isoenzymes. The MB type is specific to myocardial cells while MM and BB are specific to skeletal muscle and brain tissue respectively. The CK level will increase approximately 3-4 hours after a myocardial infarction and stays elevated for 3-4 days. This makes it useful for the detection of re-infarction in the 4-10 day time window after the initial insult since troponin remains elevated for 10 days making it less useful for this purpose.