Comparison of PPI databases. (a) Numbers of proteins in each PPI database. The bars indicate, for each of the four species, the total number of unique proteins in the six databases (DBs) before (BF: black) and after (AF: grey) standardization of identifiers (IDs). (b) Numbers of PPIs in each database before (black) and after (grey) ID standardization. (c) Numbers of PPIs present in one, two, three, or four or more databases after redundancy reduction. Human PPIs have been accessioned into all six databases, mouse PPIs into five, and fly and yeast PPIs into four. The counts include homo- as well as hetero-dimeric interactions. (d) Numbers of PPIs for which one, two, or three or more distinct literature items are cited as evidence. (e) Numbers of PPIs supported by the four high-level experimental approaches. For a-e: H (human), M (mouse), F (fly) and Y (yeast).