Additional mechanisms by which MMTVs can contribute to human breast oncogenesis include (i) proteins expressed by the MMTV envelope gene that are capable of malignantly transforming normal human breast epithelial cells [100], (ii) MMTV envelope protein p14 overexpression, which can function in an oncogenic capacity [130], (iii) MMTV encoded proteins (such as Rem, Sag, and Naf) or as yet uncharacterised proteins analogous to those of other complex retroviruses, such as Tax, may also have a role in breast cancer. This evidence concerns the gene SAG and breast cancer.