Meanwhile, mutant huntingtin (Htt), the causative gene product of Huntington's disease (HD), is known to downregulate peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC-1α) [19]–[21], a co-activator of peroxisome proliferator-activated receptor (PPAR-γ) that is essential for transcription of a number of genes involved in glucose metabolism and β-oxidation of fatty acid [for review see 22]. This evidence concerns the gene HTT and juvenile Huntington disease.