DUX4 and facioscapulohumeral muscular dystrophy: While DUX4 is notoriously difficult to detect, its molecular signature is measurable in FSHD muscle biopsies (Yao et al., 2014; Rickard et al., 2015; Banerji et al., 2017; Wang et al., 2019; Banerji et al., 2020b) and has been suggested as a disease biomarker (Wang et al., 2019).