Competing interests: Dr RG reports grants (during the conduct of the study) from NIH, The Muscular dystrophy association and The Parent Project for Muscular dystrophy, and, personal fees from Strongbridge Pharmaceuticals, Sarepta Pharmaceuticals, Marathon Pharmaceuticals, Stealth Pharmaceuticals, outside the submitted work; Dr JS reports grants from NINDS, grants from FSH Society, personal fees from Acceleron, personal fees from Sarepta, personal fees from Fulcrum, personal fees from PTC, personal fees from Dyne Therapeutics, outside the submitted work; Dr BGMvE reports grants from European Union’s Horizon 2020 research and innovation programme (Murab), grants from European FP7 programme (OPTIMISTIC), grants from Association Francaise contre les Myopathies, grants from Global FSH, grants from The Netherlands Organisation for Health Research and Development (ZonMw), grants from Prinses Beatrix Spierfonds, grants from Stiching Spieren voor Spieren, grants from Dutch FSHD Foundation, grants from Netherlands Organisation for Scientific Research (NWO), personal fees from Fulcrum, outside the submitted work. All other authors declare that they have no financial or non-financial competing interests.