HTT and juvenile Huntington disease: Longer CAG repeat mutations are associated with an earlier age of onset, more severe clinical expression and an earlier age of death.2,3 HTT appears to play an important role in neurodevelopment,4 synaptic development, neuronal survival5 and transport,6 transcriptional regulation6 and autophagy6,7; however, it is still debated as to whether Huntington’s disease results from a ‘loss of function’ of HTT or a pathological ‘gain of function’ of mHTT.