(A) Models in which macromolecules are composed of stickers and spacers can be used to predict the phase behavior of proteins (Choi et al., 2019). This schematic shows the interactions between two such macromolecules, with the stickers in one macromolecule (red shapes) forming bonds (reversible, non-covalent crosslinks) with the stickers in the other macromolecule (blue shapes); the spacers are shown as grey and black circles. Bonds between the stickers are made and broken on a time scale of tbond. (B) Free macromolecules (small orange spheres) diffuse and collide on a timescale of tdifffusion, sometimes sticking together to form condensates (large orange spheres). (C) When these two timescales are roughly equal, a phenomenon known as Ostwald ripening leads to the formation of a dominant condensate that continues to grow by absorbing smaller condensates. (D) Ranganathan and Shakhnovich predict that when the timescale for diffusion is much faster than the timescale for making and breaking bonds, condensates cannot grow beyond a certain size, which results in a large number of small- and medium-sized condensates.