Patients with MSUD, depending on the mutation, show variable degrees of enzyme deficiency leading to several different clinical phenotypes [1]. Approximately 75% of MSUD patients have the classic form of the disease with BCKDH activity in the range of 0–2% of normal [1]. These patients show markedly elevated levels of BCAA in blood and other body fluids [1]. Besides the classic form, there are other variants of the disease. Patients with the intermediate form of the disease show BCKDH activity in the range of 3–30% of normal. In such patients the onset of the disease is delayed, but there are persistently elevated levels of BCAA [1]. Patients with the intermittent form of MSUD show BCKDH activity in the range of 5–20% and during the asymptomatic phase the blood BCAA levels are normal [1]. The overall incidence of MSUD in the general population is 1:185,000 [1], and in certain population groups, such as Mennonites of Pennsylvania, the incidence is estimated to be as high as 1:176 [2].