Reflecting the above considerations, we have taken a two-stage approach to further development of the hematopoietic cells in the Cell Ontology. In the first stage, which is now complete, we revised current terms and added new terms so that all hematopoietic cell type terms now have textual definitions that contain all the necessary details to define the cells logically. These terms have been directly incorporated into the existing ontology. Fig. 2A shows a typical OBO term stanza for one of these new terms, “induced T-regulatory cell.”