The clinical phenotypes of MSUD are based on the severity of the response to the residual BCKDH enzyme activity and the gene locus affected, including classic MSUD cases, which demonstrate only 0–2% residual BCKDH activity, intermediate MSUD cases that have 2–8% activity, and intermittent MSUD cases that have 8–15% residual BCKDH activity. This evidence concerns the gene PPM1K and maple syrup urine disease.