Apolipoprotein D (Apo D) is a glycoprotein with a molecular weight of approximately 33 kDa, encoded by the APOD gene in humans. Unlike other major lipoproteins primarily produced in the liver, Apo D is mainly synthesized in the brain and testes. It is a component of high-density lipoprotein (HDL) and does not share significant sequence similarity with other apolipoproteins. Apo D is closely associated with phospholipid:cholesterol acyltransferase, which participates in lipoprotein metabolism. It also serves as an important link during transient interactions between HDL and LDL particles and between HDL particles and cells. Apo D is a biomarker for androgen insensitivity syndrome (AIS) and is associated with neuronal disturbances and nerve injury, particularly with regard to myelin sheaths. In rodent models of stroke, the levels of Apo D are elevated. Apo D levels are also increased in patients with schizophrenia, bipolar disorder, and Alzheimer’s disease.