An immense number of post-translational modifications on histone proteins have been described and additional sites of modification are still being uncovered. Whereas many direct and indirect connections between certain histone modifications and distinct biological phenomena have now been established, concepts for comprehending the extreme density and variety of these covalent modifications are lacking. Here, we formally introduce localized 'binary switches' and 'modification cassettes' as new concepts in histone biology, elucidating mechanisms that might govern the biological readout of distinct modification patterns. Specifically, our hypotheses provide missing models for the dynamic readout of stable histone modifications and offer explanations for several long-standing questions embedded in the literature. Our ideas might also apply to non-histone proteins and are open to direct experimental examination.
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