The Human Endogenous Retrovirus ERV-3 Is Upregulated in Differentiating Placental Trophoblast Cells

The single copy endogenous retrovirus locus ERV-3 is known to be primarily expressed in the placenta. The absence of expression of this gene in choriocarcinoma cell lines has led to speculation that this may be a defect associated with this abnormality. We show here that ERV-3 is not normally expressed in the cytotrophoblast from which these tumour cells are derived but is expressed in normal syncytiotrophoblast. The conservation of the ERV-3 open reading frame for env in ape and old world monkey species and its tight regulation and site of expression suggest a functional role for this gene in this tissue.