Going live: A protein‐labeling method based on the use of a single amino acid tag—an N‐terminalcysteineresidue—and small‐molecule probes containing a cyanobenzothiazole (CBT) unit has been used for the specific fluorescence labeling of proteins in vitro and at the surface of live cells (see scheme). This simple ligation reaction proceeds with a high degree of specificity under physiological conditions