Examples of neighborhood relationships and target sets. (a) Expression profiles: target sets correspond to sets of genes having similar expression profiles, i.e. nodes of the binary tree resulting from a hierarchical clustering of the profiles. (b) Chromosome localization: sets of adjacent genes correspond to the nodes of an implicit lattice resulting from the order of the genes on the chromosome. (c) Gene Ontology annotations: target sets correspond to GO terms, i.e. genes are grouped in a set corresponding to a term when they are annotated with this term or a more specific one.