Cystatin B is a protein that plays a crucial role in regulating the activity of enzymes called cysteine proteases, which are involved in various biological processes, including protein degradation and immune response. It is encoded by the CSTB gene and is found in many tissues throughout the body, particularly in the brain. Mutations in the CSTB gene can lead to a condition known as cystatin B deficiency, which is associated with neurological disorders, including progressive myoclonus epilepsy type 1. Understanding cystatin B is important for research into these conditions and potential therapeutic approaches.