TGFB1 and breast cancer: However, even though the expression of Snail transcription factors in the pulmonary endothelium (Fig. 4a–g) and TGF-β1 in the lungs (Fig. 4h) (both known to be main drivers of EndMT) were increased in relation to healthy controls already in the premetastatic stage (1st week), the onset of functional phenotypic switch of pulmonary endothelium known as EndMT seemed to take place no earlier than the 3rd week after breast cancer cell inoculation (Fig. 4i).