24-dehydro Cholesterol-d6 is intended for use as an internal standard for the quantification of 24-dehydro cholesterol by GC- or LC-MS. 24-dehydro Cholesterol is an immediate precursor to cholesterol in the Bloch pathway of cholesterol biosynthesis. Structurally, it varies from cholesterol only by a single double bond at carbon 24 and has been used as cholesterol substitute in cellular membrane studies.¹ During brain development, 24-dehydro cholesterol transiently accumulates, composing up to 30% of total brain sterol, where it is poised to rapidly enrich membrane sterols.² However, defects in cholesterol synthesis can lead to a condition called desmosterolosis, which results in an accumulation of excess 24-dehydro cholesterol.³ 24-dehydro Cholesterol has been reported to activate liver X receptor-target genes in both the liver of cholesterol-free mice and in cholesterol-starved macrophage foam cells in atherosclerotic lesions.^4,5