DUX4 and facioscapulohumeral muscular dystrophy: However, high levels of DUX4-fl expression in many skeletal myonuclei does not recapitulate the mosaic rare myonuclear expression of DUX4-fl found in FSHD subjects and preclinical testing for different candidate FSHD therapeutics targeting DUX4-fl mRNA, and protein expression will likely require different criteria, such as lesser degrees and slower progression of pathophysiology, dependent upon the approach.