Cholesterol and other sterols exit the body primarily by secretion into bile. In patients with sitosterolemia, mutations in either of two ATP-binding cassette (ABC) half-transporters, ABCG5 or ABCG8, lead to reduced secretion of sterols into bile, implicating these transporters in this process. To elucidate the roles of ABCG5 and ABCG8 in the trafficking of sterols, we disrupted Abcg5 and Abcg8 in mice (G5G8 ؊/؊ ). The G5G8 ؊/؊ mice had a 2-to 3-fold increase in the fractional absorption of dietary plant sterols, which was associated with an Ϸ30-fold increase in plasma sitosterol. Biliary cholesterol concentrations were extremely low in the G5G8 ؊/؊ mice when compared with wild-type animals (mean ؍ 0.4 vs. 5.5 mol͞ml) and increased only modestly with cholesterol feeding. Plasma and liver cholesterol levels were reduced by 50% in the chow-fed G5G8 ؊/؊ mice and increased 2.4-and 18-fold, respectively, after cholesterol feeding. These data indicate that ABCG5 and ABCG8 are required for efficient secretion of cholesterol into bile and that disruption of these genes increases dramatically the responsiveness of plasma and hepatic cholesterol levels to changes in dietary cholesterol content.ATP-binding cassette transporters ͉ sitosterolemia ͉ bile ͉ knockout mice S itosterolemia is a rare autosomal recessive disorder characterized by the accumulation of plant and animal sterols in blood and tissues (1, 2). Affected subjects with this disorder develop large deposits of cholesterol in their skin, tendons, and coronary arteries. The accumulation of plant and animal sterols in the blood is caused by an increase in the fractional absorption of sterols from the diet and a decrease in the secretion of sterols into the bile, which is the major route of exit of sterols from the body (3, 4).A striking feature of sitosterolemia is the precipitous fall in plasma cholesterol that follows reductions in dietary cholesterol intake, especially in young patients (5, 6). When normal individuals are switched from a high cholesterol, high fat diet to a low cholesterol, low fat diet, plasma levels of cholesterol fall Ϸ10-20%; in patients with sitosterolemia, plasma cholesterol can fall by Ͼ45% (5, 6). The hypercholesterolemia of sitosterolemia is also sensitive to treatment with bile-acid resins, which stimulate the conversion of cholesterol to bile acids, another pathway for removal of cholesterol from the body.The pathognomonic feature of sitosterolemia is the elevation in plasma sitosterol, the most abundant plant sterol (1). Sitosterolemic patients also accumulate other sterols in plasma including a variety of plant sterols (campesterol, stigmasterol, and avenasterol) and shellfish sterols (brassicasterol, 24-methylene cholesterol, and 22-dehydrocholesterol) (1, 7). In normal individuals these sterols are poorly absorbed and preferentially secreted into the bile (8-10). These sterols comprise only Ϸ1% of plasma and tissue sterols in normal individuals but Ϸ15% of circulating and tissue sterols in sitosterolemia (11).Studies of sitoste...