DUX4 and facioscapulohumeral muscular dystrophy: As only 3/5 TIRMPOS biopsies also had increased fat infiltration, inclusion of both fat fraction and TIRM hyperintensity as a biomarker for DUX4 signature expression, resulted in the highest sensitivity for DUX4POS biopsies [11/19 (57.9%) DUX4POS biopsies, Fig. 4c] without substantially reducing specificity [11/12 (91.6%) biomarker-positive FSHD biopsies are DUX4POS].