There are three genetically determined disorders in which one or more of the lipoprotein families are absent from plasma or their concentrations are extremely low: abetalipoproteinemia, hypobetalipoproteinemia, and Tangier disease (familial high density lipoprotein deficiency). Hypocholesterolemia is a common manifestation of all these diseases, which most easily can be distinguished on the basis of clinical symptoms and lipoprotein patterns.