ERVW-1 and hydatidiform mole: In contrast, enhanced HERV expression is associated with development and invasion of several cancers, e.g., the up-regulation of syncytin-1 promotes the malignant transformation of hydatidiform moles (32) and the spread of endometrial carcinoma (33), and all malignancies of placental origin (including those of late appearance) derive from cells of the conceptus, not of the mother.