The mouse mammary tumour virus (MMTV) is a milk-transmitted type B retrovirus. The superantigen (SAg) is encoded by the long terminal repeat. The SAgs are also called PR73.

1: Superantigens of mouse mammary tumor virus. Acha-Orbea H, MacDonald HR; Annu Rev Immunol 1995;13:459-486. PMID:7612231