Cholesterol synthesis pathway and human malformation syndromes. The sterol ring structure and carbon position numbering are shown at the top of the figure. Human malformation syndromes are in boldface type. Cholesterol is synthesized from squalene in a series of enzymatic reactions denoted by numbers: 1, squalene monooxygenase and squalene cyclase; 2, lanosterol 14-α-demethylase; 3, 3β-hydroxysterol Δ¹⁴-reductase; 4, C4 demethylation complex (C4-sterol methyloxidase, C4-sterol decarboxylase [NSDHL], and 3-ketoreductase); 5, 3β-hydroxysterol Δ⁸,Δ⁷-isomerase; 6, lathosterol 5-desaturase; 7, 3β-hydroxysterol Δ⁷-reduct-ase; 8, 3β-hydroxysterol Δ²⁴-reductase.