Apolipoproteins are a family of fatty-acid binding proteins that transport fat through the bloodstream in the form of lipoproteins. ApoO (Apolipoprotein O), also known as FAM121B or My025, is a 198 amino acid single-pass membrane protein that belongs to the apolipoprotein family. Expressed ubiquitously with particularly high expression in diabetic heart tissue, 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. Two isoforms of apoO exist due to alternative splicing events.