Another study further pointed out the positive correlation between leptin expression and proliferation pathway including leptin receptor, aromatase, mitogen activated protein kinase (MAPK) and signal transducer and activator of transcription-3 (STAT3) in breast cancer patients with estrogen receptor expression and obesity, suggesting the effect of leptin-leptin receptor axis in tumor microenvironment [25]. This evidence concerns the gene STAT3 and obesity due to melanocortin 4 receptor deficiency.