SLC6A3 and Parkinson disease: The aim of the study was to evaluate the association of individual and combined single‐nucleotide polymorphisms in brain‐derived neurotrophic factor (BDNF), dopamine transporter (DAT), and catechol‐O‐methyltransferase (COMT) genes with the occurrence of motor levodopa‐induced complications (MLIC) in Parkinson's disease (PD).