4α-Methyl-cholest-5-en-3β-ol-d3 is the labelled analogue of 4α-Methyl-cholest-5-en-3β-ol (M295715), a derivative of Cholesterol (C432501), a major component of all biological membranes; ~25% of total brain lipid is Cholesterol. Cholesterol is the principal sterol of the higher animals. Cholesterol was found in all body tissues, especial in the brain, spinal cord, and in animal fats or oils. Cholesterol is the main constituent of gallstones.