HTT and Huntington disease: To explore the role of S303 and S307 phosphorylation in HSF1 regulation in HD, HSF1 phosphoproteomics was performed in hsf1−/− MEFs inducibly expressing human Htt-Q74 or Htt-Q23 and transfected with an HSF1 expression vector (Fig. 1j and Supplementary Data 1).