Bard J, Rhee S, Ashburner M. An ontology for cell types. [Genome Biology 2005, 6:R21]: Describes an ontology for cell types that covers the prokaryotic, fungal, animal and plant worlds. It includes over 680 cell types that are classified under several generic categories and are organized as a directed acyclic graph. The ontology is is designed to be used in the context of model organism genome and other biological databases. Availability: http://obo.sourceforge.net/.