Aldotetroses

Threose is a four-carbon monosaccharide with molecular formula C4H8O4. It has a terminal aldehyde group, rather than a ketone, in its linear chain and so is considered part of the aldose family of monosaccharides. The threose name can be used to refer to both the - and -stereoisomers and more generally to the racemic mixture (/L-, equal parts D- and L-) as well as to the more generic threose structure (absolute stereochemistry unspecified).

Erythrose is a tetrose saccharide with the chemical formula C4H8O4. It has one aldehyde group, and is thus part of the aldose family. The natural isomer is D-erythrose; it is a diastereomer of D-threose. thumb|left|Fischer projections depicting the two enantiomers of erythrose