Cystatin B deficiency is a rare genetic disorder caused by mutations in the CSTB gene, which provides instructions for producing the protein cystatin B. This protein plays a crucial role in regulating enzymes that break down proteins in the brain. A deficiency can lead to an accumulation of these enzymes, resulting in damage to nerve cells. Individuals with cystatin B deficiency may experience symptoms such as epilepsy, cognitive impairment, and movement disorders. The severity of symptoms can vary widely among affected individuals. Early diagnosis and supportive care can help manage the condition and improve quality of life.