TARDBP and amyotrophic lateral sclerosis: •There was no difference between fibroblast cell growth, shape and spreading in patients and controls cells.•Mislocalization and accumulation of TDP-43 were not detected in patients and controls cells.•The cytoskeleton appeared completely normal in cells and no difference was observed in the distribution of mitochondria.•The rate of ROS production and its stress response in patient-derived cells and controls showed similarity.•Patient-derived fibroblasts are not suitable for pathological and prognostic studies of ALS.