TGFB1 and breast cancer: MMTV-PyVT mice have been crossbred with other genetically modified mice to study the effects on mammary tumor progression and metastasis of PI3K/Akt signaling [12], colony-stimulating factor-1 (CSF-1) [13], nuclear factor kappa B (NF-KB) [14], CD4+ T cells [15], transforming growth factor beta 1 (TGF-β1) [16], plasmin deficiency [17] and MEKK1 [18].