Fig. 6

Histogram of proteins with only positive interactions. Of the 3677 unique PDBs, 3453 PDBs are involved in only positive interactions, i.e., among all the protein–protein pair instances in our database, these 3453 proteins do not feature in any non-complex forming instance. Moreover, of the 3453 PDBs with only positive interactions, nearly 82% unique PDBs are involved in fewer than 4 PPI examples. Consequently, for a classifier to memorize data and not “learn” to predict interactions would be extremely difficult without each PDB appearing in a relatively large number of PPI instances