Subretinal injection is a precise method used to deliver substances such as viral vectors, stem cells, or drugs into the subretinal space. The protocol involves anesthetizing the subject (e.g., mouse or rat), dilating the pupil, stabilizing the eye, making a small scleral incision, carefully inserting a fine-gauge needle under the retina, slowly injecting the substance, and monitoring for post-injection recovery. This technique is widely used in ophthalmic research and gene therapy applications.