In an in-vivo condition, in mice, the impact of anti-HERV-K Env monoclonal antibodies (mAbs; 6H5, 4D1, 4E11, 6E11, and 4E6) on expression of HERV-K Env in the targeted cell lines, dimension, and apoptosis of xenograft tumor cells were evaluated. This evidence concerns the gene ERVW-1 and neoplasm.