25-hydroxy Cholesterol-d6 is intended for use as an internal standard for the quantification of 25-hydroxy cholesterol by GC- or LC-MS. 25-hydroxy Cholesterol is an oxysterol.¹ It is formed from cholesterol by cholesterol-25-hydroxylase, and its production can be induced by inflammation or infection.² 25-hydroxy Cholesterol suppresses endogenous cholesterol synthesis by binding to insulin-induced gene (INSIG) proteins and preventing sterol regulatory element binding proteins (SREBPs) from being transported to the Golgi. It inhibits IgA class switching induced by LPS and various cytokines in B cells (IC50 = ~50 nM).³ 25-hydroxy Cholesterol inhibits replication of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in Vero cells (EC50 = 3.675 µM) and reduces increases in viral protein production in infected Vero cells when used prior to infection.⁴ It reduces viral RNA loads in the lung and trachea in a mouse model of SARS-CoV-2 infection when administered at a dose of 100 mg/kg per day. Serum levels of 25-hydroxy cholesterol are increased in patients with SARS-CoV-2.5