Higher percentages of HERV-K Env protein expression were demonstrated in BC cell lines (Hs578T, MDA-MB-231, MCF-7, MDA-IBC-3, and BT549) than in HEK293 or HEK293T cells by flow cytometry (FACS), cell enzyme-linked immunosorbent assay (ELISA), immunofluorescence staining (IFS), immunohistochemistry (IHC), and immunoblot (data not shown). The gene discussed is ERVW-1; the disease is breast cancer.