HTT and Huntington disease: Here, we studied the diurnal and circadian rhythms and visual discrimination responses of a different model, the BAC HD mouse, which has been shown to recapitulate some of the HD pathophysiology and behavioral abnormalities [7], [8].  Although the R6/2 model is the best-characterized and most widely-used model of HD [17], [18], [19], [20], [21], research has actively focused in alternative models as the R6/2 expresses only the short N-terminal mutant fragment of the huntingtin protein.