Protein imaging with tetrazine ligation using small molecule-TCO conjugates. a, e and i chemical structure of BI 2536-TCO, AZD2281-TCO and MLN8054-TCO, respectively. b, f and j Merged image of c & d, g & h, and k & l, respectively. scale bar: 10 μm. c BI 2536-TCO/Texas Red-Tz staining (d) GFP-PLK1 in PANC-1 cells. g AZD2281-TCO/Texas Red-Tz staining. h anti-PARP 1 monoclonal antibody staining. k MLN8052-TCO/CFDA-Tz staining. l RFP-AKA in PANC-1 cell. (AKA : Aurora Kinase A. CFDA : carboxyfluorescein diacetate. GFP : green fluorescent protein. PARP : Poly(ADP-ribose) polymerase 1. PLK : Polo-like kinase 1. RFP : red fluorescent protein.) Reprinted with permission from ref 40-42. Copyright 2010-2012 John Wiley & Sons, Inc.