## 1 Introduction  
Mushrooms, belonging to the class Basidiomycetes and sometimes Ascomycetes, are a widely distributed food resource on earth and have been consumed because of their nutritional value and medicinal properties for over 2000 years. In addition to their enjoyable flavor and taste, human health was improved by mushrooms due to their nutrients, including digestible proteins, carbohydrates, fiber, vitamins, minerals, and antioxidants [1; 2]. A wide variety of bioactive compounds from medicinal mushrooms, which are widely used in eastern Asia, have been studied extensively, and these compounds including polysaccharides, lectins, lactones, terpenoids, and alkaloids have been reviewed [1; 3; 4]. Among these bioactive compounds, polysaccharides (PSs) with various activities are the main component for the bioactivities of some mushroom species. The diverse activities displayed by PSs include antitumor, immunomodulatory, anti-inflammatory, antinociceptive, antiviral, antioxidative, hypoglycemic, and hepatoprotective effects, as well as protection against chronic radiation stress [1; 2; 3; 4; 5].  
PSs are divided into two distinct categories, homopolysaccharides (homo-PSs) and heteropolysaccharides (hetero-PSs), on the basis of monosaccharide composition. Both homo-PSs and hetero-PSs may possess homolinkages or heterolinkages in regard to configuration and/or linkage position. In addition to their differing types and sequences of monosaccharide units, hetero-PSs have various types and sequences of glycosidic linkages, resulting in practically limitless structural diversity [5]. Earlier studies suggested that homo-PSs, particularly glucans, are the only PSs with medical properties. However, hetero-PSs also display important biological activities [1], and both homo-PSs and hetero-PSs bioactivities are closely related to their specific structural characteristics [5]. The detailed mechanisms of bioactivities related to structure in hetero-PSs remain largely unexplored, although the essential roles of glucans and glycans have been well established. PS composition and combination are clearly correlated with pharmaceutical activities, and this relationship has been the subject of increasing research attention [6; 7; 8].  
The production of chemically and biologically uniform PSs requires knowledge of PS synthetic pathways and regulatory mechanisms. The biosynthetic pathways of PS involve synthesis of nucleotide sugar precursors, assembly of repeating monosaccharide units, and the polymerization process. In PS biosynthetic pathways, enzymes involved in the biosynthetic pathways of nucleotide sugar precursors are closely related with production of mushroom PSs [9; 10; 11; 12]. In submerged culture of _Ganoderma lucidum_ (lingzhi mushroom), overexpression of homologous \(\alpha\)-phosphoglucumutase and uridine diphosphate glucose pyrophosphorylase (UDP-glucose pyrophosphorylase) genes enhanced PS production [13; 14]. Relationships between the monosaccharide composition of PSs and activities of related enzymes were recently described [15; 16].  
Numerous studies have addressed the structures and bioactivities of mushroom saccharides, and the relationship between them. Here, we review recent work in this field, with focus on the relationship between monosaccharide composition and the activities of mushroom PSs. We discuss: (i) monosaccharide composition/combinations in various mushroom PSs, and their relationships with bioactivities; (ii) possible biosynthetic pathways of mushroom PSs and effects of key pathway enzymes on monosaccharide composition; (iii) regulation strategies in PS biosynthesis, and prospects for controllable synthesis of PSs with enhanced bioactivities.