OPN4 encodes melanopsin, a G protein-coupled photoreceptor that serves as the primary mediator of non-image forming responses to light. The protein functions by binding cis-retinaldehydes and exhibiting peak sensitivity around 480 nm blue light 1. OPN4 operates through multiple G protein signaling pathways, including canonical Gq activation and robust Gi/Gβγ signaling, which may explain its functional diversity 2. The receptor is expressed in photosensitive retinal ganglion cells and plays crucial roles in circadian photoentrainment, pupillary light reflexes, and sleep/wake timing regulation 13. Genetic polymorphisms in OPN4, particularly I394T, P10L, and R168C variants, are associated with altered light sensitivity, sleep disorders, and seasonal affective disorder 43. Beyond the retina, OPN4 is expressed in choroidal nerve fibers, intrinsic neurons, and melanocytes, suggesting broader physiological roles 5. Recent research demonstrates that modified OPN4 can be used optogenetically to induce hibernation-like states in mice 6, and near-infrared light activation of OPN4 promotes hippocampal neurogenesis with anxiolytic effects 7. The protein's diverse signaling capabilities and widespread expression pattern support its fundamental role in light-mediated physiological processes.