To determine whether MBD3L proteins de-repress the NuRD complex-regulated D4Z4 array, we transduced control and FSHD myoblasts with a lentiviral vector delivering a doxycycline-inducible MBD3L2 transgene and, after selecting for transgene-expressing cells, analyzed DUX4 mRNA and protein after 48 hr of doxycycline treatment. Here, MBD3L1 is linked to facioscapulohumeral muscular dystrophy.