We found that HERV-K Env protein expression was significantly increased in 11 types of cancers, and knockout (KO) of the HERV-K env gene using CRISPR-Cas9 can potentially reduce tumorigenic characteristics, including proliferation, invasion, and migration, in DLD-1 colorectal cancer cells and ovarian cancer cell lines [17,18,19]. Here, ERVW-1 is linked to cancer.