DUX4 and facioscapulohumeral muscular dystrophy: However, unlike other muscle diseases, in which all of the myocytes of an affected individual share the same functional deficit [63], it is estimated that <1% of FSHD myonuclei express the pathogenic DUX4-fl mRNA at any particular time [5, 6], and that sporadic bursts of DUX4-fl expression from an increased number of myonuclei may lead to pathology [14].