Plasmids were delivered to HeLa cells with X-tremeGENE HP DNA Transfection Reagent (Roche) according to the manufacturer's instructions, and at a ratio of 2 μl X-tremeGENE HP DNA Transfection Reagent per 1 μg plasmid. Briefly, 2 μg of plasmid and 4 μl of X-tremeGENE HP DNA Transfection Reagent were added per single well of a 12-well plate, and the cells were harvested 36–48 h post-transfection. To test the splicing of cTNT e5, 200 ng of the minigene was delivered to each well. X-tremeGENE HP DNA Transfection Reagent (Mock)- and EGFP-treated HeLa cells were used as controls. For immunoblotting experiments, cells were grown in a 6-well plate and a double amount of plasmid and the transfection reagent were used. Annealed siRNA oligos were delivered to fibroblasts using Lipofectamine 2000 (Invitrogen) per manufacturer's instructions. Oligos targeting human MBNL1 (47 (link)) and MBNL2 (48 (link)) were obtained from Future Synthesis and RiboTask, respectively. The cells were harvested 72 h post siRNA delivery. Lipofectamine (mock)- and AllStars Negative Control siRNA (Qiagen; Ctrl)-treated fibroblasts were used as controls.