HSD3B2 encodes 3β-hydroxysteroid dehydrogenase type II, a bifunctional enzyme catalyzing the oxidative conversion of Δ5-ene-3β-hydroxy steroids and ketosteroids, playing a crucial role in biosynthesis of all hormonal steroid classes 1. The enzyme localizes to mitochondrial and endoplasmic reticulum membranes and operates as the primary steroidogenic isoform in adrenal and gonadal tissues, in contrast to the minimally expressed HSD3B1 in adrenal cortex 2. Loss-of-function HSD3B2 mutations cause congenital adrenal hyperplasia type 2, characterized by impaired cortisol and aldosterone synthesis with shunting of precursors toward elevated DHEA/DHEAS production and abnormal sex differentiation in both sexes 1. HSD3B2 expression is regulated by transcription factor YY1 binding within intron 1, which is essential for maximal basal promoter activity 3. The enzyme's activity can be therapeutically modulated—metformin inhibits HSD3B2 expression and activity through mitochondrial complex I inhibition, reducing androgen production relevant to polycystic ovary syndrome treatment 4. HSD3B2 and cytochrome b5 co-express in hybrid adrenocortical cells at the zona fasciculata-reticularis border, collectively contributing to androstenedione production 5. Additionally, somatic mutations in genes regulating HSD3B2 activity occur in cortisol-producing adenomas 6.