Interestingly, a growing body of evidence reveals that the expression of major genes involved in cholesterol synthesis in the brain was obviously downregulated, including DHCR24, in aged mice, diabetic mice, familial Alzheimer’s disease (FAD) mice, and AD patients, which suggest the decrease in de novo cholesterol synthesis and brain cholesterol loss [20–25]. The gene discussed is DHCR24; the disease is Alzheimer disease.