Phosphorylation of Drp1 at Ser‐616 by cyclin‐dependent kinase (CDK) 1/cyclin B or CDK5 promotes mitochondrial fission, whereas dephosphorylation of Drp1 at Ser‐637 by calcineurin facilitates its translocation to mitochondria and subsequently increases mitochondrial fission (Liesa et al, 2009; Campello & Scorrano, 2010). Therefore, a balance between Drp1 Ser‐616/Ser‐637 phosphorylation ratio reflects Drp1 activity. Western blot analysis of total protein lysates showed a significant increase in Drp1 phosphorylation at Ser‐616 combined with a decrease in phosphorylation at Ser‐637 in NSC‐34 SOD1 G93A cells (Fig 2C, P = 0.0002). These results indicate that Drp1 hyperactivation and phosphorylation occur in NSC‐34 cells expressing SOD1 G93A and that treatment with P110 inhibits this hyperactivation (P = 0.0012).