ERVW-1 and neoplasm: Furthermore, tumor xenograft assay showed that Syncytin-1 overexpression in NIH3T3 cells could induce tumor formation at the flank of nude mice, whereas down-regulation of Syncytin-1 expression in HCCLM3 cells effectively inhibited tumor formation (p < 0.001, Fig. 3f, Supplementary Fig. S1, f, Supplementary Tables S4, 5).