Protein-protein interactions (PPIs) deposited in publicly available literature derived human PPI databases. (A) Human PPIs present in interaction databases. The distribution of the number of PPIs annotated in each database is shown. For each bar, 'total' refers to the number of PPIs listed (i.e. claimed) at the database websites or number of PPIs in the downloaded datasets while the orange portion represents the number of human non-redundant direct PPIs calculated by us. (B) Distribution of the number of interacting proteins. Different scales are used to depict the number of proteins annotated with 1–10, 11–20, or 21–30 or higher number of PPIs per protein. All datasets were downloaded on October 2, 2006.