OPN4 and Huntington disease: That study reported a decrease in pupillary light responses (PLRs; or the ability of the pupil to constrict in response to light, a marker of light reception in the retina) that is correlated with downregulation of the photopigments melanopsin and cone opsin in both R6/2 mice and a full-length knock-in mouse model of HD (Ouk et al., 2016b).