4alpha-formyl-4beta-methyl-5alpha-cholesta-8,24-dien-3beta-ol is a 3-beta-hydroxysterol that is an intermediate in cholesterol biosynthesis I and in cholesterol biosynthesis III (via desmosterol). It is a substrate for C-4 methyl sterol oxidase (SC4MOL) and can be generated from the enzymatic reduction of 4alpha-carboxy-4beta-methyl-5alpha-cholesta-8,24-dien-3beta-ol or from the enzymatic oxidation of 4alpha-hydroxymethyl-4beta-methyl-5alpha-cholesta-8,24-dien-3beta-ol. The sequence of reactions and the types of intermediates in cholesterol biosynthesis II may vary. Alternate routes exist because reduction of the carbon 24,25 double bond on the hydrocarbon side chain of the sterol ring structure by sterol delta24-reductase can occur at multiple points in the pathway, giving rise to different intermediates. These intermediates, with or without a double bond in the hydrocarbon side chain, can serve as substrates for the other enzymes in the pathway. The sequence of reactions and the types of intermediates in cholesterol biosynthesis III (via desmosterol) may vary. Alternate routes exist because reduction of the carbon 24,25 double bond on the hydrocarbon side chain of the sterol ring structure by sterol delta24-reductase can occur at multiple points in the pathway, giving rise to different intermediates. These intermediates, with or without a double bond in the hydrocarbon side chain, can serve as substrates for the other enzymes in the pathway. In cholesterol biosynthesis I, 4alpha-formyl-4beta-methyl-5alpha-cholesta-8,24-dien-3beta-ol is an intermediate in the conversion of lanosterol to cholesterol. The enzymology of this multistep conversion was largely determined in rat liver and the human pathway is therefore inferred from this work. Indeed, the order of some of the reactions in this pathway may vary. The lanosterol-to-cholesterol conversion involves the oxidative removal of three methyl groups, reduction of double bonds, and migration of the lanosterol double bond to a new position in cholesterol. The reactions in the lanosterol pathway are catalyzed by membrane-bound enzymes. Human genes have been identified for all the enzymes in this pathway and human disorders of cholesterol metabolism have been associated with genetic defects in most of these enzymes.