Proteins are the main building blocks from which cells are assembled, and they constitute most of the cell's dry mass. Almost all novelty in protein structure comes from the way these single domains are arranged. The number of multidomain families being added to the public databases is still rapidly increasing, unlike the number of novel single domains. Understanding the structure of a protein at the atomic level allows us to see how the precise shape of the protein determines its function. Once a protein had evolved a stable conformation with useful properties, its structure could be modified over time to enable it to perform new functions. The union of structure, chemistry, and function gives proteins the extraordinary ability to orchestrate the large number of dynamic processes that occur in cells. The biological function of a protein depends on the detailed chemical properties of its surface and how it binds to other molecules, called ligands.