Intracerebral viral injection is a technique for delivering viral vectors into specific brain regions to study gene expression, neurodegeneration, or circuit manipulation. The protocol involves anesthetizing the subject (e.g., mouse or rat), securing it in a stereotaxic frame, performing a small craniotomy, inserting a fine-gauge needle into the target brain area using stereotaxic coordinates, slowly injecting the virus, and post-injection monitoring for recovery.