DNMT3A and thymoma: DNMT3A is a DNA methyltransferase required for de novo methylation and mammalian development.16 It is known to act as a tumor suppressor in acute myeloid leukemia (AML) and is observed mutated less frequently in other tumor types.17, 18 The nonsynonymous mutation observed in this thymoma sample leads to a nonconservative substitution of aspartate for glycine at amino-acid 728 (p.G728D) within the DNA methylase domain of DNMT3A.