Deficiency of CCR2 or its ligand, MCP-1, in mice results in failure of monocyte mobilization and is associated with protection from monocyte infiltration into adipose tissue and insulin resistance [70, 71]; Spite et al. [72] report that activation of the leukotriene B4 (LTB4) and its receptor BLT-1 axis is required for obesity-induced increases in peripheral blood monocytes and subsequent adipose tissue macrophage accumulation. This evidence concerns the gene CCL2 and obesity due to melanocortin 4 receptor deficiency.