The whole-body loss of clock function (e.g., CLOCK or BMAL1 knockout animals) leads to hyperglycemia, glucose intolerance, and, ultimately, obesity and metabolic syndrome [19], whereas ablation of the liver CLOCK impairs glycogenesis and reduces hepatic glucose production [20], indicating the varying consequences of CLOCK dysfunction are tissue-specific [21]. Here, CLOCK is linked to obesity due to melanocortin 4 receptor deficiency.