Chemically modifying proteins with imaging probes or nonnatural amino acids can be a powerful way to study biological processes. But existing methods for introducing modifications work only at high reactant concentrations and can leave unwanted amino acid “scars.” A new method developed by Tom W. Muir and coworkers at Princeton University overcomes these limitations (Nat. Chem. 2019, DOI: 10.1038/s41557-019-0281-2). Their method uses self-splicing protein segments called inteins. They made the inteins in two parts, one of which is decorated with the moeity the researchers want to add to the protein, the other of which is attached to the protein. The first half is split into two pieces, including a short “overhang” of 10 or fewer amino acids. Step 1 of the process (shown) involves reconnecting the overhang, which carries the desired modification, to the rest of the truncated intein via an enzymatic transamidation reaction. Then in step 2 (shown), the truncated intein reacts with its other half. After the two halves are reconnected, they splice themselves out, leaving the chemical modification and no more than an extra cysteine on the recombinant protein. The whole process occurs as a single-pot reaction cascade at low peptide and protein concentrations. The researchers were able to add multiple chemical modifications to a single protein. The method works with isolated proteins and with more complex structures such as cellular chromatin.