Cryptic allosteric sites-transient pockets in a folded protein that are invisible to conventional experiments but can alter enzymatic activity via allosteric communication with the active site-are a promising opportunity for facilitating drug design by greatly expanding the repertoire of available drug targets. Unfortunately, identifying these sites is difficult, typically requiring resourceintensive screening of large libraries of small molecules. Here, we demonstrate that Markov state models built from extensive computer simulations (totaling hundreds of microseconds of dynamics) can identify prospective cryptic sites from the equilibrium fluctuations of three medically relevant proteins-β-lactamase, interleukin-2, and RNase H-even in the absence of any ligand. As in previous studies, our methods reveal a surprising variety of conformations-including bound-like configurations-that implies a role for conformational selection in ligand binding. Moreover, our analyses lead to a number of unique insights. First, direct comparison of simulations with and without the ligand reveals that there is still an important role for an induced fit during ligand binding to cryptic sites and suggests new conformations for docking. Second, correlations between amino acid sidechains can convey allosteric signals even in the absence of substantial backbone motions. Most importantly, our extensive sampling reveals a multitude of potential cryptic sites-consisting of transient pockets coupled to the active site-even in a single protein. Based on these observations, we propose that cryptic allosteric sites may be even more ubiquitous than previously thought and that our methods should be a valuable means of guiding the search for such sites. molecular dynamics | native state dynamics T he degree to which standard, rational drug design protocols ignore protein-conformational changes is a potentially major flaw. Most assume that proteins are frozen in their crystallographic structures because we do not understand the intrinsic dynamics of proteins well enough to include them. Once this assumption is made, the only way to manipulate a protein's activity is with inhibitors that bind more tightly to the protein's active site than the molecule's natural ligand. Unfortunately, only approximately 15% of proteins have a sufficiently deep pocket coinciding with their active site to make this strategy feasible (1).Accounting for conformational heterogeneity could greatly expand the repertoire of available drug targets by revealing cryptic sites-transient pockets that are not readily visible in experimental structures, yet can inhibit enzymatic activity via allostery or block protein-protein interactions (2-5). For example, they may provide previously undescribed druggable pockets on proteins that are already considered viable targets, or even make it possible to target proteins that are currently considered undruggable. Targeting these sites may be easier than targeting the active site if there is no natural ligand to outcompete. Finally, crypt...