Transgenic mice with an overexpression of FGF21 have a reduction in plasma triglyceride levels [14]; however, in human subjects, FGF21 correlates positively with triglycerides suggesting that FGF21 is a result of obesity and may compensate for the metabolic stress in states of obesity and insulin resistance. Here, FGF21 is linked to obesity due to melanocortin 4 receptor deficiency.