In the study performed by Murillo-Saich et al. [61], they found an inverse correlation between myostatin serum levels, skeletal muscle mass index, and free fat mass index, while, in the study by Gonzalez-Ponce et al. [63], the group of RA patients with low muscle mass also had higher levels of myostatin, as well as a greater frequency of patients with high levels of myostatin. The gene discussed is MSTN; the disease is rheumatoid arthritis.