SLC6A3 and Parkinson disease: The study found a positive association between a combination of AG (adenine-guanine) BDNF, AG (adenine-guanine) DAT, and GG (guanine-guanine) COMT genotypes with MLIC and concluded that genetic biomarkers could help determine which PD patients are more prone to the long-term side effects of therapy with levodopa [33].