CKM and Duchenne muscular dystrophy: It is informative to compare the serum levels of creatine kinase-M (CK-M) with the Titin-N levels and Titin-N/Cr- ratios measured in this study, because CK-M was the first marker reported to be elevated in the serum of DMD patients16 and is commonly used as a blood-based biomarker for muscular dystrophy to evaluate the level of muscle damage and necrosis17, 18, 19.