Parkinsonism can also be seen in Dopa-Responsive Dystonia, a condition that results from a mutation in the GTP-Cyclohydrolase 1 gene which can lead to a defect in a cofactor for tyrosine hydroxylase and the impaired manufacture of dopa and dopamine. While it typically presents as dystonia. it can present as a biochemically based form of parkinsonism (due to reduced synthesis of dopamine) which closely resembles PD and responds to levodopa, but is not associated with abnormalities on fluoro-dopa positron emission tomography (FD-PET) nor neurodegeneration. This diagnosis should be considered in individuals aged <20 years who present with a clinical picture resembling PD. Ref Harrison 20th edition page 3123