High-density lipoproteins (HDL) form one of the main lipoprotein families present in plasma. It is a lipoprotein family of very heterogeneous particle size, apolipoprotein composition and density (which varies between 1.063 g/ml and 1.21 g/ml). Cholesterol transported by HDL is a protective factor against atherosclerosis, whereas the cholesterol present in low-density lipoproteins (LDL) constitutes a risk factor. It is therefore generally considered positive to have a high concentration of HDL cholesterol in plasma together with a low level of LDL cholesterol.

These concepts have their basis in several clinical and epidemiological studies that have demonstrated an inverse relationship between the concentration of plasma HDL cholesterol and the risk of suffering atherosclerosis, while this relationship is direct in the case of LDL cholesterol. Nevertheless, the relation-ship between HDL and atherosclerosis is complex and has exceptions. For example, patients with very infrequent genetic alterations which cause almost total deficiencies of HDL do not appear to run any greater risk of atherothrom-botic cardiovascular disease than control subjects. This feature has recently been reproduced in animals in which genes of vital HDL proteins were specifically knocked out.