To explore the efficacy of unsymmetric hydroxyaryl-H-Tz in cleaving functional C2TCO conjugates, we employed a previously developed model system. We have shown that the ligand-C2TCO-dye conjugate 22 (Ib-C2TCO-SiR) forms a covalent bond with Bruton's tyrosine kinase (BTK) through the integrated inhibitor ibrutinib (Ib, see Fig. ). Incubation of HT1080 cells stably transfected with BTK-mCherry with 22 (2 µM) resulted in selective binding of the probe to BTK, indicated by colocalization of the fluorescence signals from the silicon-rhodamine dye (SiR)