text: In organic chemistry, a tetrose is a monosaccharide with 4 carbon atoms. They have either an aldehyde () functional group in position 1 (aldotetroses) or a ketone () group in position 2 (ketotetroses).
  The aldotetroses have two chiral centers (asymmetric carbon atoms) and so 4 different stereoisomers are possible. There are two naturally occurring stereoisomers, the enantiomers of erythrose and threose having the  configuration but not the  enantiomers. The ketotetroses have one chiral center and, therefore, two possible stereoisomers: erythrulose (- and -form). Again, only the  enantiomer is naturally occurring.