Here we show that global ablation of Bmal1 in Apoe−/− and Ldlr−/− mice and its liver-specific ablation in Apoe−/− (L-Bmal1−/−Apoe−/−) mice increases, whereas overexpression of BMAL1 in L-Bmal1−/−Apoe−/− and Apoe−/−mice decreases hyperlipidaemia and atherosclerosis. The gene discussed is APOE; the disease is hyperlipidemia.