The acyl pyridinium intermediate derived from DSH reacted only with nearby Lys residues. The molecular dynamics (MD) simulation indicates that the distance between the acyl carbon atom of the acyl pyridinium intermediate and the N-terminal nitrogen atom of LANA is 4.3~10.4 Å (Figure ). Thus, PEG-LANA-DSH acylated Lys residues within the reach of this distance, called catalyst effective region (CER). Indeed, the MD simulation based on the co-crystal structure of nucleosome-LANA -DSH (PDB: 5GTC) suggests that both H2BK120 and H2BK116 were located within CER. Despite the significant difference in acetylation yield, averaged distances between these Lys residues and the N-terminus of LANA were comparable each other: 9.2 Å for H2BK120 and 8.3 Å for H2BK116 (Figures 1b, 1c, and Figure ). Thus, other factors than the proximity to the activated acetyl pyridinium may also be involved in the Lys residue selectivity.