The expression of HERV-K, a biologically active family of HERV, produces proteins found in placenta. Furthermore, the expression of the envelope genes of HERV-W ([https://www.genenames.org/data/hgnc_data.php?hgnc_id=13525 ERVW-1] ) and HERV-FRD ([https://www.genenames.org/data/hgnc_data.php?hgnc_id=33823 ERVFRD-1] ) produces syncytins which are important for the generation of the syncytiotrophoblast cell layer during placentogenesis by inducing cell-cell fusion. The HUGO Gene Nomenclature Committee (HGNC) approves gene symbols for transcribed human ERVs.