Therefore, the aim of this study was to investigate the potential association of selected polymorphisms, located in the HTR2C, HTR6 and SLC6A3 genes, coding for 5-HT2CR, 5-HT6R and DAT, respectively, with the haloperidol-induced parkinsonism in schizophrenia patients, by using another well-validated rating scale, namely, the Extrapyramidal Symptom Rating Scale (ESRS) [40,41]. The gene discussed is SLC6A3; the disease is Parkinson disease.