Lipoprotein lipase (LPL) is an enzyme which plays a major role in the metabolism of circulating triglyceride-rich lipoproteins. It hydrolyzes chylomicron and VLDL triglycerides, thereby delivering fatty acids to tissues for storage or oxidation. In order to gain insight into the molecular basis of LPL deficiency, the structure of the LPL gene (ten exons and nine introns spanning about 30 kb) is first set out in relation to the different domains of the LPL protein. There is a high sequence homology between the aminoacids of LPL and of other lipases, such as hepatic triglyceride lipase (HL) and pancreatic lipase (PL). The PL catalytic triad Ser132, Asp156, His241 is also present in LPL. Absence of LPL activity can result from absence of LPL protein synthesis (Brunzell class I), or from the synthesis of an LPL protein devoid of enzymatic activity consequently to a mutation (class II). LPL can also be unable to bind to endothelial cells--a defect combined with deficient enzymatic activity--(class III). Among the known mutations of the LPL gene (such as nonsense, frameshift, abnormality in intron-exon junction, deletion, duplication) resulting in pathological cases, the most frequent are punctual mutations located mainly in exons 4, 5 and 6, leading to the substitution of an aminoacid for another in essential domains of LPL. The combined deficiency LPL + HL has also been described. The study of the abnormalities of the LPL gene, known only since the years 1990-1991, allows not only to better understand the pathology of LPL deficiencies, but also to point out which aminoacids play a major role in LPL activity.