MX1 and obesity due to melanocortin 4 receptor deficiency: To test the role of myeloid cell intrinsic Jak2–Stat3 signaling inducing obesity-associated insulin resistance, we generated control (Mx1-Cre/Jak2+/+) mice and mice with the hematopoietic cell ablation of Jak2 (Mx1-Cre/Jak2−/−).