We noted that DUX4-FL expression frequencies in these three cohorts inversely correlated with D4Z4 array length as measured by EcoRI-BlnI restriction fragment length (Table 1 and Additional file 1: Table S1), which, despite the limited sample size, is potentially intriguing considering short 4q D4Z4 arrays (<5 RUs) are associated with severe FSHD disease while longer arrays show more inter-individual variation in clinical severity [20,25]. This evidence concerns the gene DUX4 and facioscapulohumeral muscular dystrophy.