The mechanisms by which they contribute to oncogenesis are diverse and include: (i) insertional mutagenesis, due to activation of cellular proto-oncogenes by inserted proviruses, (ii) immunosuppression, by an immunosuppressive domain conserved in most retroviral envelope proteins and (iii) direct oncogenic activity, with some retroviruses encoding proteins with transforming activities leading to tumour formation. Here, ERVW-1 is linked to neoplasm.