The goal of the present study was to investigate if a common polymorphism (-579G>T: rs1569686) in the promoter of the DNMT3B gene coding for the DNA methyltransferase 3B, one of the key de novo enzymes mediating DNA methylation, increases the risk to develop MG or MG-associated thymomas. This evidence concerns the gene DNMT3B and myasthenia gravis.