LEP and obesity due to melanocortin 4 receptor deficiency: Studies going back to the 1990s have implicated leptin signaling as a positive regulator of HSC proliferation and differentiation (32–34), thus raising the question of the developmental impacts on hematopoiesis prior to the onset of obesity in an environment devoid of functional leptin or leptin receptors, as in the ob/ob and db/db mice, respectively.