Representative Western blots showing siRNA‐mediated reduction in total Fis1 levels. n = 3.Mitochondrial elongation in healthy control fibroblasts and ALS patient‐derived fibroblasts from the stained TOM20 images was quantified using a macro in ImageJ after Fis1 siRNA treatment. n = 3 performed in duplicate. At least 100 cells/group were analyzed while blinded to experimental conditions.Mitochondrial interconnectivity in healthy control fibroblasts and ALS patient‐derived fibroblasts from the stained TOM20 images was quantified using a macro in ImageJ after Fis1 siRNA; n = 3 performed in duplicate. At least 100 cells/group were analyzed while blinded to experimental conditions.Representative Western blots showing Drp1 and p62 association with mitochondrial‐enriched fractions in different ALS patient‐derived fibroblasts following culturing in the presence or absence of P110 (1 μM for 24 h); VDAC was used as a loading control for mitochondrial fraction; n = 3.Levels of p62 in mitochondrial fractions were measured by immunoblotting; VDAC was used as a loading control. Protein levels were quantified and presented as fold change of control 1. Results are presented as fold of control (means ± SD). n = 3 performed in triplicate; probability by one‐way ANOVA (with Tukey's post hoc test).Representative Western blots showing Drp1 and Fis1 total levels in different ALS patient‐derived fibroblasts following culturing in the presence or absence of P110 (1 μM for 24 h); β‐actin was used as a loading control; n = 3.Drp1 association with Fis1 was examined by Western blot following co‐immunoprecipitation of pooled total lysates from three independent experiments from different ALS patient‐derived fibroblasts following treatment in the presence or absence of P110 (1 μM for 24 h). n = 3.Data information: Mean, standard deviation, and P‐values are shown.Source data are available online for this figure.