SAG and breast cancer: Additional mechanisms by which MMTVs can contribute to human breast oncogenesis include (i) proteins expressed by the MMTV envelope gene and which are capable of malignantly transforming normal human breast epithelial cells,29 (ii) MMTV envelope protein p14 overexpression can function in an oncogenic capacity30 (iii) MMTV encoded proteins (such as Rem, Sag, Naf) or as yet uncharacterised proteins analogous to those of other complex retroviruses such as Tax may also have a role in breast cancer.