PAQR9 (mPRε) is a plasma membrane progesterone receptor coupled to G proteins, functioning primarily through G(s)-mediated signaling pathways 1. Beyond progesterone, PAQR9 binds multiple neurosteroids including DHEA, pregnanolone, pregnenolone, and allopregnanolone 1, and may regulate rapid steroid signaling in the nervous system 2. Mechanistically, PAQR9 modulates protein quality control by regulating BAG6-mediated triage of mislocalized membrane proteins, reducing their polyubiquitination and degradation through competitive binding to BAG6's DUF3538 domain 3. During fasting, PAQR9 stabilizes PPARα protein by competing with the E3 ubiquitin ligase HUWE1, thereby promoting hepatic ketogenesis and fatty acid oxidation 4. PAQR9 expression is regulated by viral infection and aging; varicella zoster virus infection alters PAQR9 expression in the ventral tegmental area, with age-dependent effects on pain responses 5. Genome-wide association studies identified PAQR9 as associated with plasma phospholipid concentrations 6. In cancer contexts, PAQR9 expression is altered in head and neck squamous cell carcinoma and hepatocellular carcinoma, with potential prognostic implications 78. Additionally, genetic variation in PAQR9 correlates with immune cell populations implicated in chr3 kidney disease pathology 9.