MSTN and proximal spinal muscular atrophy: BMD, IBM, and FSHD patients, who clinically show a less pronounced muscle atrophy, have higher circulating myostatin than DMD and SMA patients but less than controls (BMD 71% ± 23.7, IBM 71% ± 55.6 and FSHD 66% ± 35 compared to controls, respectively).