metastasis. Here Py is the abbreviation of polyoma and MT is the abbreviation for middle T. There are more model systems of breast cancer which use the MMTV promoter. The polyoma middle T-antigen is taken from the polyoma virus. The MMTV-PyMT model has been shown to be a reliable model of breast cancer metastasis. In human breast cancer the polyoma middle T- antigen was not found. Notes References Betaretroviruses Rodent diseases Animal viral diseases