Numerous molecular variants of the protein moiety of human circulating lipoproteins ("apolipoproteins" or "apoproteins") have been described in recent years. Molecular alterations of apolipoproteins may lead to an impaired lipid binding and/or to an accelerated or delayed lipoprotein catabolism. Many variants, particularly those of the E apoprotein system, are associated with premature atherosclerosis. In the case of the Apo AI variants, the concomitant deficiency of Apo AI and Apo CIII leads to severe clinical atherosclerosis. Conversely, molecular variants of Apo AI (several of which come from FRG, i.e. AI-Marburg, -Giessen, -Münster) do not go together with significant clinical abnormalities. The case is different for Tangier disease, characterized by the complete absence of high density lipoproteins, where a dramatic tissue lipid deposition may occur. One molecular variant, Apo AI-Milano, while leading to a significant reduction of HDL, does not seem to be associated with clinical atherosclerosis, but rather with a protection from the disease. The presence of major apolipoprotein abnormalities in familial groups of variable size, provides a molecular explanation for some significant alterations of lipid metabolism. Moreover, it offers, to clinical and basic studies, a useful model for the understanding of the function and metabolism of human apolipoproteins.