An aldohexose is a hexose with an aldehyde group on one end.
The aldohexoses have four chiral centres for a total of 16 possible aldohexose stereoisomers (2). Of these, only three commonly occur in nature: D-glucose, D-galactose, and D-mannose. The D/L configuration is based on the orientation of the hydroxyl at position 5, and does not refer to the direction of optical activity.