Science tends to be the study of what we can study. Its focus is biased toward those aspects of the world for which we have experimental and conceptual tools. It’s not just that we are limited by our tools, but that this limitation skews our perception of how the world, and how life, works.

The vast majority of protein structures obtained so far are for proteins that form crystals—which is by no means all of them. Our portraits of proteins are not representative. Among the proteins that don’t yield to structural analysis are many that simply don’t have well-defined folded shapes. Instead, parts of their polypeptide chains are loose and floppy.

Intrinsically disordered proteins are not strongly committed to a particular “shape”. It is not possible for disordered proteins to engage with other molecules with lock-and-key specificity. Many of them will bind to all manner of other molecules, including other proteins. Often a binding event gives such proteins a more orderly, folded form: they “fold on binding” into a shape that is contingent on the partner. Other disordered proteins remain resolutely unstructured even when they bind, latching on amoeba-like to form sprawling, fuzzy unions.

Fuzziness and imprecision are not only common; they are features of some of the most important molecular unions in the cell. Many of the proteins that play central roles in orchestrating molecular events in our cells seem explicitly shaped by evolution to be promiscuous, binding several similar partners with more or less equal avidity. This is not a mistake or shortcoming of evolution, but an intentional feature.

The disorder and promiscuous binding of many human proteins doesn’t just mean that the molecular mechanisms of our cells are a bit messier and looser than was thought. It implies that the whole logic of how information flows within and between cells needs rethinking.

The flexibility conveyed by disorder underscores how important changes in shape are for the way these molecules work. The very notion of “protein structure” is perhaps better regarded as a set of shapes akin to the choreographed poses through which a dancer continually moves.

Philip Ball, How Life Works