Migration of monocytes into extravascular sites, including adipose tissue, and their differentiation into macrophages that elaborate cytokines, including TNFα and IL-6, which impair insulin sensitivity, are thought to represent a critical series of events in the development of diet-induced insulin resistance in diabetes and obesity and to involve tissue elaboration of the cytokine Monocyte Chemoattractant-1 (MCP-1) and its interaction with the monocyte MCP-1 receptor CCR2 [31,32,33,34,35,36]. The gene discussed is CCR2; the disease is obesity due to melanocortin 4 receptor deficiency.