We also studied LIF expression and its ability to induce Stat3 activation in mouse mammary tumor models with different grades of differentiation and malignancy: the non-metastatic mouse mammary tumor virus (MMTV)-induced hormone-dependent and hormone-independent neoplasias (HDTs and HITs, respectively) [24,25] and the poorly differentiated highly metastatic LM3 mammary tumor line [26]. This evidence concerns the gene STAT3 and breast cancer.