CDK5 and Huntington disease: In a transgenic (Tg) HD mouse model that expresses exon 1 of the human HTT gene, with approximately 125 CAG repeats (R6/2) (115), a knock-in chimeric HD mouse model, that expresses a human HTT exon 1/ mouse Htt with 140 CAG repeats (KI140) (116), as well as in patients, the levels and activity of CDK5 and GSK-3β have been found to be dysregulated (55, 56), suggesting that redirecting kinase inhibitors levels/activity could potentially abrogate tau hyperphosphorylation observed in HD.