The NIAID workshop attendees discussed how best to characterize hematopoietic/immune cell subsets, as well as how to improve the overall ontological structure of the corresponding portions of the CL and of the CL ontology in general. The consensus view was that the current multiple inheritance structure of the CL is unsustainable and that existing and new terms for hematopoietic cells should be logically defined via their structural parts and qualities as represented in other ontologies. Much discussion centered on what might be the optimal axis of differentia for these hematopoietic terms. It was recognized that, in many cases, these cell types are defined largely, but not solely, by the expression of particular marker proteins either at the cell surface (e.g. receptor proteins) or internally (e.g. transcription factors). The presence of these proteins as part of a cell is considered a structural feature of the cell, and we have chosen to use the relation has_part (or an appropriate sub-relation) from the OBO Relation Ontology (RO) to relate particular cell types to protein terms from the Protein Ontology (PRO) or protein complex terms from the cellular component ontology of the Gene Ontology (GO) [7] and [8].