Structure publications linked by Medical Subject Headings (MeSH) terms. (a) All available primary citations for Protein Structure Initiative (PSI) structures (shown in blue) were compared with primary citations from a random set of 300 Protein Data Bank (PDB) structures (shown in yellow). Blue color nodes are segregated into tight clusters, indicating close similarity among PSI structures. Yellow nodes are relatively interspersed because of fewer common edges, indicating greater variability in connecting MeSH terms. Out of a total of 860 MeSH terms associated with PSI structure nodes (435 of which are unique), the term 'Bacterial proteins - chemistry' is the most common, connecting a clique of 52 nodes. (b) As a control, two random sets of 300 PDB structures were chosen from a set of 3,112 structures released in 2001-2002 that included a primary citation available in PubMed. Those sets of citations were then run through PubNet as Query1 and Query2. Nodes of both colors are fairly well interspersed. The PDB structure nodes carry 1,247 MeSH terms (861 unique), the most common of which is also 'Bacterial proteins - chemistry', but it only connects a clique of 16 nodes.