 Patients have markedly reduced whole-body cholesterol biosynthesis associated with suppressed hepatic, ileal, and mononuclear leukocyte hydroxymethylglutaryl-coenzyme A reductase (HMG-CoA reductase), the rate-controlling enzyme in the cholesterol biosynthetic pathway.Whether or not the down-regulation is due to accumulated sitosterol is still debatable, but most recent data indicate that secondary effects of unknown regulators other than sitosterol can lead to reduced HMG-CoA reductase activity in the disease.This is coupled with significantly increased low-density lipoprotein (LDL) receptor expression.