Apolipoproteins are a family of fatty-acid binding proteins that transport fat through the bloodstream in the form of lipoproteins. ApoO functions to promote the transport of cholesterol from macrophage cells and may be involved in regulatory mechanisms that protect lipid accumulation within the heart. ApoO is present in high density lipoproteins (HDLs) and low density lipoproteins (LDLs), and is secreted by an MTP (microsomal triglyceride transfer protein)-dependent mechanism. ApoOL (apolipoprotein O-like), also known as FAM121A, is a 268 amino acid secreted protein belonging to the apolipoprotein O family and may be involved in cholesterol transport. ApoOL is encoded by a gene located on human chromosome X, which contains nearly 153 million base pairs and houses over 1,000 genes.