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 a side-chain substituted oxysterol derived from dietary cholesterol that inhibits the cleavage of sterol regulatory element binding proteins (SREBPs) to suppress endogenous cholesterol synthesis in various cell types.¹ It has been implicated in a variety of metabolic events including cholesterol homeostasis and atherosclerosis as well as antitumor activities as it has been shown to induce apoptosis through down-regulation of Bcl-2 expression and activation of caspases.² Immunomodulating capabilities have also been observed as the oxysterol can act as a LXR-RXR ligand coupling cholesterol synthesis to T cell proliferation, can reduce (EC50 ~ 65 nM) IgA production by B cells in response to IL-2, and can suppress differentiation of monocytes into macrophages.^3,4,5