Only one one-way MR study that examined causality between serum iron levels and PD suggested a causal relationship between elevated iron levels (instrumental variables, HFE rs1800562, HFE rs1799945, and TMPRSS6 rs855791) and a reduced risk of PD (OR 0.88, 95% CI 0.82–0.95; p = 0.001; Pichler et al., 2013). Here, HFE is linked to Parkinson disease.