FRG1 and facioscapulohumeral muscular dystrophy: To test the hypothesis that the premature activation of the FRG1 gene during the differentiation of FSHD cells may have been due to a weaker repression mechanism mediated by the contracted D4Z4 allele, we compared H3K27 trimethylation levels in the D4Z4 repeats in control and FSHD myoblasts before and after myogenic differentiation.