The modification of nucleosome N-terminal tails by small molecules. Modifications are shown on just one tail of each type of core histone. Residue numbers are given for the modified residues (here, shown by one-letter abbreviations and subscript numbers—S₁, K₅, K₉, etc.). The N-terminal tail is on the left, connecting to the globular domain (indicated as a triangle), followed by the short C-terminal tail. Modifications occur at all these positions, but the most common are those in the N-terminal tails. The major modifications are represented as follows: P, phosphorylation; A, acetylation; M, methylation; and U, ubiquitination.