COS-7 cells expressing wild-type or mutant alleles of ATP1A3 were assessed in the ATPase assay as described previously (4 (link)). Briefly, synthesized adenosine 5′-diphosphate (ADP) by ATPase reaction was detected using the EnzyLight ADP Assay Kit (BioAssay Systems, Hayward, CA, USA). ATPase activity was estimated by subtracting the luminescence signal after a 1-hour pretreatment with 100 μM ouabain (Sigma-Aldrich, St. Louis, MO, USA) from the luminescence reading without ouabain.