Difucosyllacto-N-hexaose b is a fucosyl-oligosaccharide from human breast milk. For several years biochemical studies on the carbohydrate composition of human milk have concerned only the dosage of lactose. In fact, even though oligosaccharides had already been identified at the end of the 19th century, the methods available at that time did not allow further data to be obtained on their precise amounts. The availability of more advanced techniques has recently made it possible to establish the precise amount of oligosaccharides in human milk during the different phases of lactation. The results obtained so far have quantified the concentration of such substances at 20 to 23 g l-1 in colostrum and 12 to 14 g l-1 in mature milk; thus, from the quantitative point of view, oligosaccharides represent the third component of human milk, following lactose and lipids. However, even though improvements have been made regarding the total amount of oligosaccharides, knowledge on the amount of each oligosaccharide in human milk remains inadequate. From the biochemical point of view, such substances result from the sequential addition of monosaccharides to the molecule of lactose, by specific glycosyltransferases of the mammary gland. In particular, monosaccharidic units represented by fucose (Fuc), galactose (Gal), N-acetylglucosamine (GlcNac) and sialic acid (NANA) are added to lactose, forming oligosaccharides of different dimensions, with a linear or ramified molecular structure. Such a synthetic process leads to the formation of a significant number of oligosaccharides. More than 80 oligosaccharides have already been identified which, on the basis of their chemical composition, can be classified as follows: (i) "core" oligosaccharides, made up of Glc, Gal and GlcNac, representing the starting structures for the synthesis of more complex oligosaccharides; (ii) fucosyl-oligosaccharides, deriving from the addition to the core of one or more molecules of Fuc; (iii) sialyloligosaccharides, resulting from the addition to the core of one or more molecules of NANA; and (iv) sialylfucosyl-oligosaccharides, containing both Fuc and NANA. The presence and the quantity of different types of oligosaccharides in human milk are genetically determined and closely related to the secretor state A, B, H and Lewis, with significant differences among the different genotypes. The most common phenotype, present in 70% of the general population, is represented by subjects expressing both the secretor and Lewis gene. The milk produced by these mothers is the richest in oligosaccharides. It is evidently difficult to analyse oligosaccharides of human milk as they are numerous and have a particularly complex structure. The data available until now demonstrate that the majority of human milk oligosaccharides is made up of molecules with three to nine monosaccharidic units (trisaccharides to nonasaccharides). (PMID:10569230).