Leptin can bind to Ob-R on breast cancer cells and enhance several tumor cell responses in tumor tissue by inappropriately activating multiple signaling pathways, such as MAPK and ERK1/2 activation, signal transducer and activator of transcription 3 (STAT3), and phosphatidylinositol 3-kinase/protein kinase B (PI3K/Act) [61]. Here, STAT3 is linked to breast cancer.