25-Hydroxycholesterol is a steroid derivative that suppresses the cleavage of sterol regulatory element binding proteins (SREBPs). It inhibits HMG-CoA reductase and so it also plays a significant role in the in vivo regulation of cholesterol biosynthesis after an acute dietary cholesterol challenge. 25-hydroxycholesterol is a potent (EC50≈65 nM) and selective suppressor of IgA production by B cells.