Co-occurrence frequency matrix of 27 histone modifications determined by mass spectrometry experiments. Histone modifications located within the same histone tail were identified by mass spectrometry. Raw frequency counts of histone modification pairs were normalized by the maximal observed co-occurrence count of histone modification pairs. Normalized matrix was clustered using hierarchical clustering with complete linkage; Pearson correlation of the normalized co-occurrence counts was used as the similarity measure. The block on the upper left corner contains histone 3 modifications and the block on the bottom right corner contains histone 4 modifications.