We first characterized two hTERT-immortalized myoblast cell lines derived from the deltoid muscles of a female FSHD patient (12A) and her unaffected sibling (12U), by assessing expression levels of DUX4-fl and target genes TRIM43 and MBD3L2. As expected, we found significantly high levels of DUX4 and target gene expression in the patient myoblasts as compared to the unaffected control (Fig. 3a; see Supplementary Table S2 online). Here, MBD3L2 is linked to facioscapulohumeral muscular dystrophy.