In the present study we investigated the methylation levels of genes involved in DNA methylation reactions, such as DNMT1, DNMT3A, and DNMT3B, and in one-carbon metabolism, MTHFR, in blood and tumor tissue DNA from 69 thymoma-associated myasthenia gravis patients; for 44 of them we also had thymic epithelial tissue adjacent to the tumor. The gene discussed is DNMT3B; the disease is neoplasm.