MAO, a principal enzyme of dopamine metabolism that has two isoforms: MAO-A and MAO-B, was tested for its supposed role in PD pathogenesis: a polymorphism in intron 13 of MAO-B was significantly associated with increased risk of PD (Kang et al. 2006; Tan et al. 2000b). This evidence concerns the gene MAOA and Parkinson disease.