DUX4L9 and facioscapulohumeral muscular dystrophy: DUX4 only being expressed in rare cells, we took advantage of the large number of FSHD muscle biopsies available here (7 patients, see Table S4) and performed immunofluorescence with the mouse MAb 9A12 antibody we had raised against DUX4 [5] on sections adjacent to the ones used above for DUX4c.