AKR1C4 is a cytosolic aldo-keto reductase that catalyzes NADPH-dependent reduction of ketosteroids to hydroxysteroids with broad substrate specificity 1. It functions as the major liver-specific enzyme for steroid hormone inactivation, demonstrating the highest catalytic efficiency among AKR1C isoforms with kcat/Km values exceeding other isoforms by 10-30-fold 1. AKR1C4 plays a critical role in the 'backdoor' androgen biosynthesis pathway, working with AKR1C2 to convert 5α-dihydroprogesterone to allopregnanolone (3α,5α-THP), which is essential for embryonic testicular differentiation and male sex determination 2. The enzyme regulates neurosteroid concentrations by reducing steroid precursors to neuroactive compounds that modulate GABAAR-mediated neural excitability 3. In androgen catabolism, AKR1C4 predominantly inactivates 5α-DHT and progesterone, though it can also participate in phase II metabolism by reducing conjugated androgens 4. Additionally, AKR1C4 converts the pesticide chlordecone to its alcohol form, promoting biliary excretion and reducing neurotoxicity 5. Disease relevance includes associations with breast cancer risk through genetic variants affecting enzyme stability 6, bipolar disorder with mood irritability in males through progesterone metabolism alterations 7, and age at menarche timing in African-ancestry populations 8. Cell-type-specific expression in hepatocytes is regulated by HNF transcription factors 9.