It is an interesting exercise to surf the blogosphere and other, similarly reliable sources on the Internet in search of the origin of this saying. It's a Spanish quote. It's Russian, Arabic, Mexican. One blogger gives credit to his mother. Several websites assure us that the famous Greek bard Euripides proclaimed it in his tragedy Phoenissae (c. 410 bc) about a group of Phoenician women caught up in a fight among Oedipus’ sons for dominion over Thebes. As Euripides said, “every man is like the company he is wont to keep.” Some bloggers attribute the quote to Confucius’ Words of Wisdom. Others go even further back, crediting the saying to the Assyrians. It seems that we may never know the true origin. But whatever its long history, the widespread usage of this morsel of wisdom points to the universality of the deep human appreciation of companionship and the unique character that bonds a group of friends or associates. Whether it is membership of a club, guilt by association, Benjamin Franklin's pithy comment during the signing of the Declaration of Independence that “we must all hang together, or assuredly we shall all hang separately,” or the astounding popularity of social media: the company one keeps defines one's being and identity.

The observation that connectivity contains genuine information has not escaped the attention of systems biologists, who are utilizing this relationship within the realm of biological networks. We are well aware that nothing in biology occurs in a vacuum and that every component is connected to many others. The investigative challenge derives from the fact that biological networks and their connections are often only vaguely known. Because it is the task of scientists to discover the unknown, the rationale is the following: if the connectivity within a human group contains information about its members, the same could be true for biological networks. If so, it should be possible to extract novel information regarding unknown biological components from the analysis of better known components with which they associate.

The best-characterized associations and networks among biomolecules are formed by metabolites, due to two facts. First, biochemists had been working on metabolites long before DNA was identified and proteins could be manipulated with any efficiency. In fact, one of the important roots of systems biology is the work of nineteenth- and twentieth-century biochemists who formulated mathematical models for chemical reactions in metabolism.