Similar as that observed in normal human fibroblasts (Fig 5H–5L), depletion of mutant HTT expression in HD cells by siHTT led to a parallel reduction of endogenous HAP40 protein (Fig 8G–8I), while conversely HAP40 knockdown by siHAP40 over a 72 hour period resulted in a parallel depletion of mutant HTT protein as well (Fig 8J–8L). Here, HTT is linked to Huntington disease.