Because there are no biological markers for ET or PS, the diagnosis of each is based on the clinical history and findings.16, 26, 27, 28, 29 There is no diagnostic brain pathology in most ET cases,16, 17, 30 but most PS variants have distinct brain pathology.9, 10, 11, 31 Functional imaging, such as PET and dopamine transporter (DaT), studies can distinguish ET from PS cases, but cannot differentiate between PS variants.32, 33 These imaging studies are valuable for research, but are not widely available and are not essential for general neurology practice.34 The gene discussed is SLC6A3; the disease is essential thrombocythemia.