CYP7B1 is a cytochrome P450 monooxygenase that catalyzes 7-alpha hydroxylation of oxysterols and steroids, using molecular oxygen with electrons from NADPH via cytochrome P450 reductase 1. The enzyme preferentially hydroxylates steroid substrates at the 7-alpha position, particularly those with a 3-hydroxy group, and hydroxylates oxysterols including 25-hydroxycholesterol toward 7-alpha hydroxy derivatives that facilitate conversion to bile acids 1. CYP7B1 is expressed in liver, brain, reproductive tract, and immune tissues, where it regulates distinct physiological processes 2. Via its product 7-alpha,25-dihydroxycholesterol, a ligand for GPR183, CYP7B1 regulates immune cell migration and B cell maturation 3. The enzyme also metabolizes neurosteroids like dehydroepiandrosterone and pregnenolone involved in hippocampal memory and learning 4. Loss-of-function mutations cause congenital bile acid synthesis defect 3 (BASD3), characterized by neonatal cholestasis and hepatomegaly, treatable with chenodeoxycholic acid 5, and spastic paraplegia type 5, a progressive neuropathy 2. Recent studies demonstrate CYP7B1 deficiency paradoxically suppresses neuroinflammation in multiple sclerosis models and reduces macrophage infiltration during viral infections 63.