HTT and Huntington disease: Since the wild-type (Q21) and polyglutamine-expanded (Q78) HTT bind and bundle F-actin similarly in our in vitro experiments, and previous work showed that loss of HTT disrupted the axonal growth (10, 22), it is likely that the haploinsufficiency of wild-type HTT function contributes to axonal growth defects in HD, in addition to the mutant form of HTT interfering with actin regulators in a specific spatiotemporal manner.