Two healthy control‐derived fibroblasts and three ALS patient‐derived fibroblasts, each with a different genetic form of ALS, were treated with or without P110 (1 μM/24 h) for 48 h in defined medium or Fis1 siRNA and then stained with anti‐TOM20 (a marker of mitochondria, 1:500 dilution). Side panels show enlarged areas of the white boxes. Scale bar: 0.5 μm.Mitochondrial interconnectivity in healthy control fibroblasts and ALS patient‐derived fibroblasts from the stained images was quantified using a macro in ImageJ.Mitochondrial elongation in healthy control fibroblasts and ALS patient‐derived fibroblasts from the stained images was quantified using a macro in ImageJ.Mitochondrial membrane potential was determined using TMRM dye after 48 h in control and ALS patient‐derived fibroblasts in the presence or absence of P110 (1 μM/24 h).Mitochondrial ATP levels were in control and ALS patient‐derived fibroblasts in the presence or absence of P110 (1 μM/24 h).Measurement of mitochondrial ROS production using MitoSOX in ALS patient‐derived fibroblasts in the presence or absence of P110 (1 μM/24 h).Measurement of total cellular ROS production in control and ALS patient‐derived fibroblasts in the presence or absence of P110 (1 μM/24 h).Levels of Drp1 were examined in mitochondrial fractions by immunoblotting; VDAC was used as a loading control. Protein levels were quantified and represented as fold change of control 1.Association of Drp1 with Fis1 was analyzed following co‐immunoprecipitation by Western blot analysis in pooled total lysates of patient‐derived cells treated in the presence or absence of P110 (1 μM/24 h) from three independent experiments. Protein levels were quantified and presented as ratio of Drp1 to Fis1.Data information: Mean, standard deviation, and P‐values are shown. Results are presented as percent of control. n = 3 performed in (H, I) duplicate, (B, C) triplicate, or (D–G) quintuplet; probability by one‐way ANOVA (with Tukey's post hoc test). At least 100 cells/group were analyzed while blinded to experimental conditions in panels (B and C).