Apolipoprotein C4 (APOC4) is present in the APOE, APOC1, APOC4 and APOC2-gene cluster which is induced by natural and synthetic ligands of liver X receptors (LXRα, NR1H3 and LXRβ, NR1H2) in both human and mouse macrophages (Mak PA et al. 2002). The induction of all four mRNAs was greatly attenuated in macrophages derived from LXR α/β-/- mice (Mak PA et al. 2002). Cell reporter assays suggest that the LXR response elements (LXRE) in the multienhancer regions ME.1 and ME.2, which confer tissue-specific expression in macrophages and adipocytes (Shih SJ et al. 2000), are necessary for the expression of this gene cluster (Mak PA et al. 2002). These secreted apolipoproteins regulate lipid transport and catabolism.