Significance Immune formyl peptide receptors (Fprs) evolved in rodents from expression in immune cells to be transcribed in olfactory sensory neurons. Explaining the initial neuronal acquisition, we found that an Fpr coding exon landed in front of a vomeronasal receptor promoter, hijacking its expression pattern. This type of gene shuffling occurred twice in the mouse lineage, many million years apart, leading to the exclusive expression of Fprs in the two main populations of vomeronasal sensory neurons. Finally, we demonstrate that the immune expression of one of the mouse vomeronasal Fprs can be restored via the production of an intergenic transcript. Thus, we provide the complete history of genomic events that led to a model case of evolutionary neofunctionalization in a mammal.