In fact, accumulating reports suggest leptin as being a key candidate linking obesity and cancer, including: (a) leptin promotes colon cancer progression and aggressiveness [7,40], cell proliferation and tumor growth [17,18]; (b) leptin promotes mammary tumors in obese mice [41]; (c) leptin receptor’s expression is increased in tumor tissues and is necessary to promote tumor progression [42]; and (d) leptin and leptin receptor levels are used to indicate breast cancer progression [43]. Here, LEP is linked to obesity due to melanocortin 4 receptor deficiency.