The fact that DNA is wrapped around a spool, comprised of histones, is believed to influence essentially all aspects of chromosome biology, including DNA replication, repair after damage and segregation. Diverse post-translational modifications (e.g. acetylation, methylation, and phosphorylation) of histones are known and are believed to play key roles in regulating a wide swath of biology linked to our genomes. Based on observed antagonisms and synergies between different histone post-translational modifications (or 'marks') in recruiting proteins to chromosomes, it has been proposed that these 'marks' form a 'code' for regulating chromosome function. It has also been suggested that this 'code' may provide a basis of epigenetic inheritance, which is the transmission of cellular traits that are not encoded at the level of DNA sequence. Many of the proteins that post-translationally modify histones (i.e. 'write' or 'erase' the 'code') have been characterized. In contrast, our knowledge of the proteins that recognize (or 'read') histone post-translational modifications remains incomplete. The difficulty in identifying these effector-proteins (or 'readers') is, in large part, due to the histone modifications being sub-stoichiometric, dynamic, and mediators of weak interactions. With the goal to fill this knowledge gap, we have recently reported an approach, which combines photo-chemical crosslinking with bio-orthogonal chemistry, to 'capture' proteins that bind histone H3 trimethylated at Lys-4. We now combine this method with state-of-the-art mass spectrometry to develop a robust chemical proteomics approach to profile 'readers' of histone methylation 'marks.' Our ongoing work suggests that our approach is general and can be used to analyze these post-translational modification-dependent protein-protein interactions in any human cell type (e.g. normal or cancer), cell state (e.g. mitosis) or context (e.g. drug-treated). Based on these and other unpublished preliminary data, we propose to: (i) comprehensively profile proteins that recognize methylation 'marks' on histones, (ii) characterize how proteins that recognize methylation 'marks' control down-stream biology, and (iii) examine how interplay between histone phosphorylation and methylation ensures error-free chromosome segregation during cell division. We combine chemistry, biochemistry, high-resolution microscopy and cell biological approaches to gain insight into fundamental cellular processes. Our findings may reveal how improper 'reading' of histone post-translational modifications can result in disease. In the long-term, our findings may also provide a basis for developing new therapeutic strategies that target 'readers' of histone methylation 'marks'.