ERVFRD-1 (Syncytin-2) is an endogenous retroviral envelope protein that functions as a major fusogen mediating trophoblast syncytialization during placental development 1. As a transmembrane fusion protein, ERVFRD-1 recognizes cellular receptors (particularly MFSD2A) and catalyzes membrane fusion between mononucleated cytotrophoblasts to form the multinucleated syncytiotrophoblast (STB), the specialized cell layer governing maternal-fetal nutrient exchange 2. ERVFRD-1 expression is transcriptionally regulated by TFEB downstream of mTORC1 nutrient-sensing signaling; under nutrient stress, TFEB directly activates ERVFRD-1 to promote compensatory STB formation and enhance placental adaptation 3. Clinically, ERVFRD-1 expression correlates with proper placental development and fetal growth; reduced ERVFRD-1 expression occurs under metabolic stress conditions modeling maternal diabetes and obesity, impairing STB formation and hormone secretion 4. ERVFRD-1 suppression also occurs with FOXO1 inhibition during early trophoblast differentiation, preventing progression toward syncytiotrophoblast lineage commitment 5. Dysregulation of ERVFRD-1 expression by xenobiotics (e.g., valproic acid) impairs syncytialization and associates with adverse neonatal outcomes 6. These findings establish ERVFRD-1 as essential for placental formation and adaptation to nutrient availability during pregnancy.