Post-translational modifications of histone proteins, the basic building blocks 
around which eukaryotic DNA is organized, are crucially involved in the 
regulation of genome activity as they control chromatin structure and dynamics. 
The recruitment of specific binding proteins that recognize and interact with 
particular histone modifications is thought to constitute a fundamental 
mechanism by which histone marks mediate biological function. For instance, 
tri-methylation of histone H3 lysine 9 (H3K9me3) is important for recruiting 
heterochromatin protein 1 (HP1) to discrete regions of the genome, thereby 
regulating gene expression, chromatin packaging, and heterochromatin formation. 
Until now, little was known about the regulation of effector-histone mark 
interactions, and in particular, of the binding of HP1 to H3K9me3. Recently, we 
and others presented evidence that a "binary methylation-phosphorylation switch" 
mechanism controls the dynamic release of HP1 from H3K9me3 during the cell 
cycle: phosphorylation of histone H3 serine 10 (H3S10ph) occurs at the onset of 
mitosis, interferes with HP1-H3K9me3 interaction, and therefore, ejects HP1 from 
its binding site. Here, we discuss the biological function of HP1 release from 
chromatin during mitosis, consider implications why the cell controls HP1 
binding by such a methylation-phosphorylation switching mechanism, and reflect 
on other cellular pathways where binary switching of HP1 might occur.