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. The gene discussed is FRG1; the disease is Facioscapulohumeral dystrophy.