LEP and breast cancer: Such pathways that enhance breast cancer cell proliferation and inhibit apoptosis include (1) increased levels of circulating estrogen, for example, estradiol [52, 54, 57], (2) higher levels of insulin [58, 59], (3) decreased level of circulating adiponectin [60], (4) increased plasma leptin concentration [60], and (5) increased production of proinflammatory cytokines, such as interleukin-6 and tumor necrosis factor alpha [61].