Maple syrup urine disease (MSUD) is an inherited disorder named for the maple syrup or burned sugar smell from affected infants' urine and earwax. This disease is caused by lack of the enzyme (BCKAD) needed to process certain amino acids found in all foods containing protein. These amino acids and other substances build up in the blood and can cause serious damage to the brain. Infants affected by the most severe form of MSUD show poor feeding, vomiting, and lack of energy within the first several days of life. If untreated, death can occur within the first few months after birth. With early and lifelong treatment involving a special diet, individuals may develop normally and live a normal life span. However, despite careful treatment, there is an increased risk of learning problems, mental disability, and death. Infants with less severe forms of MSUD may appear normal at birth and have normal growth, but careful life and diet management is still critical to lifelong survival. MSUD is a condition that is tested for in states that have expanded newborn screening. MSUD type 1B is caused by pathogenic variants in the BCKDHB gene, making up the beta subunit of the BCKAD enzyme.