Tools for characterizing proteins: circular variance, mutual proximity, chameleon sequences and subsequence propensities Abstract: For the characterization of various aspects of protein structures, four useful concepts are discussed: chameleon sequences, circular variance, mutual proximity and a subsequence-based foldability score. These concepts were used in estimating foldability of globular, intrinsically disordered and fold-switching proteins, properties of protein-protein interfaces, quantifying sphericity, helping to improve protein-protein docking scores, and estimating the effect of mutations on stability. A conjecture about the Achilles' heel of proteins is presented as well.