MBD3L2 and Facioscapulohumeral dystrophy: To further validate that BETi function primarily by inhibiting the expression of DUX4 as opposed to affecting DUX4’s ability to induce target genes, we ectopically expressed DUX4 in 54-6 control (non-FSHD) myoblasts, added BETi at concentrations up to 20-fold higher than needed to block DUX4 expression in FHSD myoblasts, and measured endogenous levels of the DUX4 target gene MBD3L2. BETi, even at high concentrations, did not block the ability of DUX4 to transactivate MBD3L2 (Additional File 6: Figure S4).