Apolipoprotein D (APO D), a glycoprotein involved in the human plasma lipid transport system. It is a progesterone binding glycoprotein of 24,000 dalton monomer molecular size, is a constituent of high density lipoprotein in plasma. The function of apolipoprotein D in the metabolism of plasma lipoproteins is unclear but the observation that this protein forms complexes with lecithin: cholesterol acyltransferase has led to the suggestion that apolipoprotein D may be involved in cholesterol ester