To ensure that pre-clinical efficacy studies (e.g., knock-down treatments of the mutant HTT protein in HD animal models) are adequately powered to detect the predicted change in expression levels using the expanded polyglutamine MSD assay, we have performed a statistical power analysis of the assay for BAC HD, zQ175 KI and R6/2 HD mouse models (Figure S6). The gene discussed is HTT; the disease is ring chromosome 6.