an enzyme that catalyzes the phosphorylation of creatine by ATP to form phosphocreatine. It occurs as three isozymes (specific to brain, cardiac and skeletal muscle, respectively). Each isozyme has two components composed of muscle (M) and brain (B) subunits - CK1 (BB) is found primarily in brain, CK2 (MB) in cardiac muscle and CK3 (MM) primarily in skeletal muscle. Differential determination of isozymes is used in clinical diagnosis.