SWEDD subjects in the ELLDOPA study lacked clinical responsiveness to levodopa,4 and follow-up dopamine transporter (DAT) scans after 4 years remained normal.5 Recently, it was shown that abnormality in cortical plasticity, assessed by paired associative stimulation, was markedly different in PD and tremulous SWEDD.6 Alternative diagnoses have been considered as essential tremor (ET), depression, vascular or psychogenic parkinsonism, dopa-responsive dystonia, supranigral parkinsonism,4 and primary adult-onset dystonic tremor.7–9. This evidence concerns the gene SLC6A3 and essential thrombocythemia.