SLC6A3 and Huntington disease: Unlike numerous neurological diseases for which staging schemes have been developed (e.g., AD, PD, HD), the diagnosis of ET is a purely clinical one and there are no blood, cerebrospinal fluid or neuroimaging-based means to track the progression and neurodegeneration as there is in AD (amyloid positron emission tomography [PET]), in PD (dopamine transporter or fluorodopa PET scan), or HD (magnetic resonance imaging).