Are three-dimensional structures of proteins relevant in the study of cancer? The knowledge of the three-dimensional structure of a protein is crucial to gain a full understanding of its function, and structural determination has already shown its potential for guided drug design. The knowledge of the structures of proteins and their complexes with other biological macromolecules helps to elucidate functional networks and provide a better understanding of the functionally relevant behaviour of the molecular machinery of the cell. To study the cell, we must be able to work with proteins, to elucidate how they diffuse and move, to know their interacting partners, and to understand the changes induced by those interactions. Three-dimensional structures give us a picture of the protein and thereby the opportunity to introduce mutations that alter its affinity and specificity for other interactions helping us to understand the physico-chemical mechanisms that control their function. In turn these can lead to the development of novel therapies.