Desmosterol is an immediate precursor of cholesterol in the Bloch pathway of sterol synthesis and an abundant membrane lipid in specific cell types. Desmosterol has become of particular interest in the pathogenesis of Alzheimer's disease (AD) because of the report that the activity of the gene coding for the enzyme Δ24-dehydrocholesterol reductase (DHCR24), which metabolizes desmosterol to cholesterol, is selectively reduced in the affected areas of the brain. Any change in the pattern of C27 sterol intermediates in cholesterol synthesis merits investigation with respect to the pathogenesis of AD, since neurosteroids such as progesterone can modulate the tissue levels. The significant elevation of plasma desmosterol levels was revealed in patients with desmosterolosis, a rare disorder of cholesterol biosynthesis, which is caused by mutations in DHCR24. In addition, was shown, that desmosterol can be oxidized by Cytochrome P450 46A1, which is expressed in brain and as a consequence, the formation of that oxysterols in the brain could be related to the Smith-Lemli-Opitz syndrome, desmosterolosis, and other relevant diseases, as well as with signal transduction by lipids.