Photoreceptor that binds cis-retinaldehydes (PubMed:19793992). Contributes to pupillar reflex, photoentrainment and other non-image forming responses to light (PubMed:12808468). May be involved in the optokinetic visual tracking response (PubMed:26392540). May be involved in the regulation of retinal hyaloid vessel growth and regression (PubMed:30936473). Expressed in the retinal pigment epithelium and ganglion cell layer (at protein level) (PubMed:10632589, PubMed:30240620, PubMed:31607531). Also expressed in amacrine cell layers of the retina (PubMed:10632589). Weakly expressed in vibrissae, and tail (PubMed:31607531). Observed with processes in the outer strata of inner plexiform layer (IPL) close to the inner nuclear layer (INL) or is found to be bistratified with processes located both in the inner (ON) or outer (OFF) layers of the IPL (at protein level) (PubMed:19793992). A second population of isoform 1 is identified in processes which are confined to the inner layer of the IPL near to the ganglion cell layer (GCL) (at protein level) (PubMed:19793992). About 40 times more abundant than isoform 1 in the retina (at protein level) (PubMed:19793992). Isoform 2 is involved in processes localized to the outer IPL or is bistratified with processes in both the inner and outer layers of the IPL (at protein level) (PubMed:19793992). Isoform 2 is absent in the processes confined only to the inner layer of the IPL (at protein level) (PubMed:19793992). Expressed in the inner retina at postnatal day 5 (P5), and expressed in retinal ganglion cells at P12. Mice fail to show a pupillar reflex, photoentrainment of the circadian clock and other non-image forming responses to light (PubMed:12808468). Newborn mice show normal hyaloid vessel numbers and normal vessel cellularity, however vessel numbers are increased by P8 (PubMed:30936473). In Opn4 and Pde6b double knockout mice optokinetic visual tracking response is abolished (PubMed:26392540). Belongs to the G-protein coupled receptor 1 family. Opsin subfamily.