Acyl-COA: cholesterol acyl transferases or ACAT is an enzyme that catalyzes the esterification of cholesterol to form cholesteryl ester. Minimally, ACAT-mediated formation of cholesteryl ester from cholesterol prevents the toxic accumulation of excess cholesterol in a cell and maintains a free diffusion gradient across the cell membrane, particularly in the small intestine. In addition, the assembly and secretion of Apolipoprotein-B containing lipoproteins in the liver and intestines is thought to be dependent on the ACAT-mediated formation of cholesteryl esters from cholesterol. In steroidogenic tissue such as the adrenal glands, ACAT activity produces cytosolic droplets loaded with cholesteryl esters from which they can be mobilized as cholesterol substrates for the generations of steroids. Furthermore, macrophages that accumulate cholesteryl ester in cytosolic lipid droplets as a result of ACAT activity appear foamy and are a characteristic early indicator of atherosclerotic lesions. Animal models that completely lack ACAT protein are viable, albeit with tissue-specific reductions in cholesteryl ester, suggesting that another ACAT enzyme is present in these animals.