One of the mechanisms through which leptin causes breast cancer progression is the regulation of breast cancer stem cell (CSC) activity by modulating the many signalling pathways (i.e., Notch, Wnt, mTOR, STAT3, HER2/Erb, and IGF pathways) and transcription factors (i.e., HIF-1 and NFκB) [65,66,67,68,69]. This evidence concerns the gene ESR2 and breast cancer.