CYP27B1 (1α-hydroxylase) is a mitochondrial cytochrome P450 enzyme that catalyzes the rate-limiting activation of vitamin D metabolism 1. The enzyme hydroxylates 25-hydroxyvitamin D3 (calcidiol) at the C1α-position to produce 1,25-dihydroxyvitamin D3 (calcitriol), the hormonally active form that binds the vitamin D receptor (VDR) to regulate calcium and phosphorus homeostasis 1. CYP27B1 mechanistically inserts molecular oxygen into substrates using electrons from NADPH via electron transport proteins, also processing vitamin D2 and inactivation pathway intermediates 1. Beyond renal calcium regulation, CYP27B1 is expressed throughout immune tissues where its activity is independently regulated by type II interferon (IFN-γ), promoting local synthesis of active vitamin D that suppresses pro-inflammatory TH1 responses and induces regulatory CD4+ T cell differentiation 23. CYP27B1 polymorphisms (rs10877012, rs4646536) associate with reduced autoimmune endocrine disorder susceptibility 4, while rare variants (rs118204012) correlate with tuberculosis susceptibility and vitamin D insufficiency 5. Defective CYP27B1 regulation occurs in breast cancer, reducing VDR signaling with tumor-suppressive implications 6. Loss-of-function variants cause vitamin D-dependent rickets type 1A, highlighting the enzyme's critical physiological role.