Approximately 12 % of histone H2B in mammalian brain contains an unusual d-aspartate residue in its N-terminal tail. Most of this d-aspartate is linked to the C-flanking glycine via an isopeptide bond. To explore the possible significance of these modifications, we generated an antibody to the d-isoaspartyl form of H2B, and used it to assess its levels in H2B associated with “active” vs. “silent” chromatin. We found that the d-isoaspartyl form of H2B appears to be highly enriched in the former. This irreversible modification could serve a novel regulatory function in gene expression.