HNMT and Parkinson disease: Despite the fact that our study has as the main limitation the relatively low number of studies on the association between HNMT rs11558538 SNP and PD risk that fulfill inclusion criteria, our data point at a protective role of the HNMT rs11558538T variant on the risk of developing PD (the calculated statistical power for the mean OR for carriers of the minor allele—0.65—seems to be acceptable), and give support to the hypothesis of a possible role of histamine in the pathogenesis of this disease.