In two subsequent studies on the large cohort of the de novo PD patients from the Parkinson’s Progressive Markers Initiative (PPMI), median raphe serotonin transporter (SERT) availability, assessed with 123I-FP-CIT SPECT, showed inverse correlations with tremor severity in the entire cohort of PD patients and in the tremulous subgroup [70, 71•]. This evidence concerns the gene SLC6A4 and Parkinson disease.