Fig. 6. Image of the hierarchy presumed for three component saccharides, glucose, mannose and galactose. Glucose (mostly in the form GlcNAc) is most fundamental, and mannose is second to it. On the other hand, galactose is distinct from these saccharides in both chemical and biological aspects (see text). Sialic acids and L-fucose are rather optional, but actually play critical roles as modifiers of galactose.