Louis Sullivan (1856 - 1924) revolutionized architecture by designing the first skyscraper and he became famous by proclaiming that "form follows function". When x-ray crystallographers visualized the structures of proteins for the first time, the structural biology field embraced the view that "function follows form" as the 3D-architecture of proteins could unveil various aspects of their function. Despite the original "1 gene - 1 protein structure - 1 function" relationship, nowadays a far more complicated picture emerges where the flexibility and dynamics of a protein can play a central role in a multitude of functions. The ultimate form(s) that a protein adopt when interacting with (a) partner molecule(s) are the most biologically relevant and in this context Sullivan's quote is still appropriate: the conformation that the protein adopts follows from the function of that protein. Despite the fact that many well-characterized proteins have a well-folded structure, there is a growing interest in the conformational flexibility within proteins. This flexibility is also a balanced phenomenon: excess of flexibility can be detrimental for protein behaviour, as well as the lack thereof. Notwithstanding its importance, studying intrinsically disordered protein regions or conformational rearrangements can be a very challenging. Therefore, flexibility can be perceived as a friend or a foe, depending on the context. This e-book showcases the impact of the study of protein flexibility on the structural biology field and presents protein flexibility in the context of disease as well as its |