Steroid biosynthesis is the set of enzyme-catalyzed metabolic pathways that generate sterols and their steroid-derived products from isoprenoid precursors. In the canonical eukaryotic pathway, acetyl-coenzyme A is converted through the mevalonate pathway into activated isoprene units, which are condensed to form squalene; squalene epoxidation and oxidosqualene cyclization then establish the tetracyclic sterol nucleus ^[1][2]. Subsequent reactions—including demethylation, reduction, oxidation, isomerization, side-chain modification, and hydroxylation—produce lineage-specific membrane sterols, such as cholesterol, phytosterols, or ergosterol, and, where present, downstream steroidal metabolites ^[2][3]. In animals, steroid biosynthesis also encompasses the conversion of cholesterol into steroid hormones through compartmentalized reactions involving cholesterol side-chain cleavage, hydroxylation, dehydrogenation, and related transformations catalyzed principally by cytochrome P450 enzymes and hydroxysteroid dehydrogenases ^[3]. The concept therefore includes both construction of the sterol scaffold from isoprenoid metabolism and its enzymatic diversification, but excludes ribosomally encoded peptide synthesis and non-isoprenoid pathways yielding structurally unrelated cyclic lipids.