PRRG2 and major depressive disorder: And even if the direct targeting of H3 serotonylation (or any other histone PTM for that matter) in human brain for the treatment of MDD or other affective disorders proves difficult given its critical baseline functions as a permissive chromatin modification, it may be possible to target its writer enzyme, TMG2, or other interacting proteins (e.g., potential readers, which have yet to be identified) in order to indirectly affect the mark’s control over aberrant stress-induced gene transcription.