HTT and Huntington disease: Taken together, this indicates that MAPK11/HIPK3-mediated HTT modulation is dependent on the presence of mHTT proteins: in WT cells without mHTT or in HD cells in which mHTT is knocked down, MAPK11/HIPK3-mediated regulation of HTT is largely absent and thus the HTT level is insensitive to these kinases (Figures 2 and 3C); in HD cells with mHTT or in WT cells with exogenously expressed mHTT-exon1 fragments, the MAPK11-mediated and HIPK3-mediated HTT regulation is enabled, and both the mHTT and wtHTT levels become sensitive to these kinases (Figures 2, 3A and 3B).