Recently, the serum creatinine-to-cystatin C ratio (Scr/Scys), also known as the sarcopenia index (SI), is suggested as a surrogate marker of muscle mass in patients with type 2 diabetes mellitus (T2DM), chronic obstructive pulmonary disease (COPD) and cancer [12–14]. The gene discussed is CST3; the disease is chronic obstructive pulmonary disease.