EIF2AK3 and Classical progressive supranuclear palsy: In contrast, reconstitution of Perk‐deficient cells with a hypomorphic PERK haplotype implicated in progressive supranuclear palsy (PSP; PERKPSP; Hoglinger et al, 2011; Yuan et al, 2018) did not impact basally fragmented mitochondria or rescue Tg‐induced mitochondrial elongation.