Methods and compositions relating to diagnosing and treating a VMAT-2 deficiency disease are described. Provided are methods for screening for, diagnosing or detecting a risk of developing a VMAT-2 deficiency disease comprising detecting the presence of a VMAT-2 variant in a sample of a subject, wherein the presence of the VMAT-2 variant is indicative that the subject has a VMAT-2 deficiency disease or an increased risk of developing a VMAT-2 deficiency disease compared to an individual having wild type VMAT-2. Also provided are methods of treating a VMAT-2 deficiency disease with a dopamine agonist.
