Need for new cell types indicated during generation of computable definitions. (a) The CL class "hematopoietic stem cell" and its computable definitions with the PRO ontology. New is_a subtypes of hematopoietic stem cell are created to represent species-specific expression of protein types. Abbreviations for protein types are used for clarity. (b) General hematopoietic progenitor cell types with characteristics shared by many species appear in the middle. Sub-types of progenitor cells classified by human-specific cell surface markers appear on the left and sub-types with mouse-specific cell surface markers appear on the right. Note that a developmental lineage of species-specific cell types can be determined by following the develops_from relationships.