Figure 1. A model of MUP action.

The MUP family members are expressed mainly by the liver and secreted into the bloodstream. MUPs bind to various volatile pheromones or other lipophilic small molecules, and regulate the transportation and bioactivity of these small molecules. MUPs and MUP-bound pheromones are excreted into the urine and detected by the main and accessory olfactory systems of conspecifics. MUPs are highly polymorphic, and the MUP profiles in urine are recognized as an identity signature of the owners by receivers. Additionally, circulating MUPs and MUP-bound bioactive molecules also regulate metabolism by suppressing the hepatic gluconeogenic and/or lipogenic programs as well as by promoting mitochondrial biogenesis and function and insulin sensitivity in skeletal muscles.