Due to the dynamic changes and the important roles of epigenetic modifications in PGC development and of the differential histone modification levels in PGCs and pluripotent stem cells, we hypothesized that abnormal changes in histone modification in Dnd1-mutant PGCs and later embryonic germ cells are involved in the initial steps of teratoma formation, i.e. the conversion of germ cells into pluripotent teratoma-forming cells. The gene discussed is DND1; the disease is teratoma.