Maple syrup urine disease (MSUD) is a potentially deadly disorder that affects the way the body breaks down three amino acids: leucine, isoleucine, and valine. When theyre not being used to build a protein, these three amino acids can either be recycled or broken down for energy. They are normally broken down by six proteins working together as a complex called BCKD (branched-chain alpha-ketoacid dehydrogenase). People with MSUD have a mutation that renders one of the 6 proteins in the complex deficient. Therefore, they cant break down leucine, isoleucine, and valine. They end up with dangerously high levels of these amino acids in their blood, causing the rapid degeneration of brain cells and, if left untreated, even death. Defects in any of the six subunits of the BCKD protein complex can cause MSUD. The most common defect is caused by a mutation in a gene on chromosome 19 that encodes the alpha subunit of the BCKD complex (BCKDHA).