Although AKT1-mediated phosphorylation of HTT (Ser421) has been well characterized in the context of scaffolding roles for kinesin-1 and dynein during axonal transport [38], it remains unknown whether ERK1 or GSK3α/β can phosphorylate HTT and how these phosphorylation events contribute to neuronal dysfunction in HD. Here, MAPK3 is linked to Huntington disease.