The hierarchical structure of the WPO. (a) The five children of the root term 'Variant' as viewed in OBO-Edit , the ontology editing tool in use at WormBase. (b) Each of these five terms (classes) has multiple descendants, as illustrated by the children and grandchildren of the 'behavior variant' term. The '+' sign in the box denotes that descendent terms are present. Clicking on the "+" sign in OBO-edit reveals the subclasses. The lowercase 'i' icon denotes the 'is_a' parent-child relationship between terms. (c) Under 'movement variant', 'locomotion reduced' is a visible subclass. Among its descendants are 'paralyzed' and 'sluggish' (see text for details). (d) On the right is the OBO-Edit display of the 'bacterially unswollen' phenotype class including a unique identifier (ID), primary name (name) and the definition of the term with references (Dbxrefs i.e., database references). The references in this case are a specific WormBase curator (cab is Carol A Bastiani) and a paper reference . Below the definition are synonyms for this term. In this case, 'Bus' is a three-letter synonym familiar to the C. elegans community. On the left is the placement of 'Bus' in the WPO. Note it has two parents, 'pathogen resistance increased' and 'tail morphology variant'.