We recently observed a reliable phenotypic difference in the inflammatory pain sensitivity of a congenic mouse strain compared to its background strain. By constructing and testing subcongenic strains combined with gene-expression assays, we provide evidence for the candidacy of the Yy1 gene – encoding the ubiquitously expressed and multifunctional Yin Yang 1 transcription factor – as responsible. To confirm this hypothesis, we used a Cre/lox strategy to produce mutant mice in which Yy1 expression was ablated in Nav1.8-positive neurons of the dorsal root ganglion. These mutants also displayed reduced inflammatory pain sensitivity on the formalin test. Further testing of pain-related phenotypes in these mutants revealed robustly increased sensitivity to systemic and spinal (but not supraspinal) morphine analgesia, and greatly increased endogenous (swim stress-induced) opioid analgesia. None of the known biological roles of Yin Yang 1 were suggestive of such a phenotype, and thus a novel player in pain modulatory systems has been identified.