The major step of regulation of cholesterol synthesis is:

Cholesterol is obtained from the diet as well as by de novo synthesis. Although many cells can synthesize cholesterol, the liver is the major site of its production. The rate of cholesterol production is highly responsive to feedback inhibition from both dietary cholesterol and synthesized cholesterol. Feedback regulation is mediated by changes in the activity of 3-hydroxy-3-methylglutaryl-CoA reductase. This enzyme is the first committed step in the production of cholesterol from acetyl-CoA. 3-Hydroxy-3-methylflutaryl-CoA, the substrate of the reductase, also can be synthesized into the ketone by acetoacetate by the action of 3-hydroxy-3-methylglutaryl-CoA lyase Ref: Essentials of Medical Biochemistry; With Clinical Cases, 1st Ed, page 215