We examined whether rat MUPn (major urinary protein n) plays a role in communication among conspecifics.Rattus norvegicus (Lewis strain) were used in this study.Here,qRT-PCR and SDS-PAGE assays revealed that mRNA for MUPn in the liver and MUPn content in urine were largely male-biased,reduced by castration and increased by testosterone administration.This implied that the urine-borne MUPn was a possible male pheromone component in rats.We cloned rat MUPn gene,and purified it by expressing a recombinant fusion protein between maltose-binding protein (MBP) and rat MUPn in Escherichia coli (rMUP-RNn) .When recombinant MUPn was added to either water or urine from a castrated male it enhanced sexual attractiveness to female rats,suggesting MUPn was a conclusive pheromone component.MUPn had synergetic effects with volatile pheromones and other MUP components,which may be used as rat attractants.