However, DUX4 is notoriously difficult to detect in FSHD, with expression reported to be as low as 1/1000–1/5000 myoblasts and 1/200 nuclei in differentiated myotubes, and in only 50% of FSHD muscle biopsies (23–26). Here, DUX4 is linked to facioscapulohumeral muscular dystrophy.