Most importantly, the reduced innervations of M1 ipRGCs, which was assessed by X-gal staining in R6/2-OPN4<sup>Lacz/+</sup> male mice, contributed to the diminished light-induced c-fos and vasoactive intestinal peptide in the suprachiasmatic nuclei (SCN), which may explain the impaired circadian photoentrainment in HD mice.