In HD mitochondrial dysregulation plays a significant role in pathogenesis (Oliveira et al., 2006) and as mentioned, SIRT1 regulates the expression of PGC-1α, a key regulator of mitochondrial function and biogenesis (Figure 4; Nemoto et al., 2005b; Chaturvedi et al., 2009). The gene discussed is PPARGC1A; the disease is Huntington disease.