Cholesterol-d7 is intended for use as an internal standard for the quantification of cholesterol by GC- or LC-MS. Cholesterol is a major sterol produced in mammalian cells that is required for cell viability and proliferation.¹ It is a component of mammalian cell membranes that interacts with membrane phospholipids, sphingolipids, and proteins to influence their behavior. It is also a component of various lipid-based drug delivery (LBDD) systems, including liposomes and lipid nanoparticles (LNPs), where it has a role in membrane stability.² Cholesterol is a precursor of steroid hormones, bile acids, and the active form of vitamin D. Impaired cholesterol homeostasis is related to the development of various diseases including fatty liver, diabetes, gallstones, dyslipidemia, atherosclerosis, heart attack, and stroke.³