STAT1 and neoplasm: To test whether these results stemmed from an intrinsic action of STAT1 within the mammary epithelium rather than simply being related to tumor-promoting changes triggered by the impaired immune system of the Stat1−/− animals, we transplanted Stat1−/− mammary epithelial tissue into the cleared fat pad of the 4th mammary gland of Stat1+/+ mice, and vice versa (see scheme in Fig. 1D).