Some association studies suggested a possible relationship of the risk of developing ET with the methylentetrahydrofolate reductase, alpha-synuclein, CYP2C19, and CYP2C9/8 polymorphisms, whereas others did not find any association with alpha-synuclein, CYP2D6, alcohol-dehydrogenase 2 (ADH2), glutathione-transferase P1 (GSTP1) (revised in reference [1]), and paraoxonase 1 (PON-1) [18] polymorphisms. This evidence concerns the gene CYP2D6 and essential thrombocythemia.