BDNF and Huntington disease: Although repeated peripheral administrations of fibroblast growth factor-2 into mouse models of HD have shown some efficacy [16], delivery of many other NTFs, including BDNF, into the HD brain remains a challenge, due to the fact that many NTFs are large, polarized proteins that do not readily cross the blood-brain-barrier [17], relegating the delivery of NTFs directly into the central nervous system.