ERVW-1 and infection: Methylation-regulated targets in hESCs (Figure S4C) include two retrovirus-derived placental genes: ERVW-1 (syncytin)68, which is critical in placental cell fusion and ERVH48–1 (suppressyn), which protects placental cells from infection by Type D retroviruses69 (Figure 3J, 4K, S4D).