This strategy found that histone monoaminylation is highly enriched in cancer cells due to an overexpression of TGM2.[131] Most recently, our lab has also reported the labeling of histone serotonylation using a pH‐controlled regioselective rapid ene‐type reaction (RRER).[132] Utilizing alkynyl TAD‐based probes, serotonylated H3Q5 peptide mimic could be successfully labeled, expanding our understanding of the selectivity of TAD‐mediated RRER. This evidence concerns the gene TGM2 and cancer.