The analysis indicated that three peptides (YSVVCELVGK, NITCDLLEHQYWLK, and YGNGHGLNATAVHCYTK) had been modified by a single neutral monoisotopic mass 482.195405, corresponding to the covalent adduct product of compound 4 (C28H26N4O4). Of these three peptides, the relative abundance of one (YSVVCELVGK) was orders of magnitude higher than the other two (6.2 x 10 8 vs 1.0 x 10 7 and 1.9 x 10 7 ). In order to confirm the modification of this peptide, the peptide was isolated and fragmented using collision induced dissociation (CID). Fragmentation confirmed the peptide sequence and located the modification to the residue Cys368. CDK11 (CDK19) and CDK8 which have residues equivalent to C368 were then evaluated using the KINOMEscan TM technology. Kinase-tagged T7 phage strains were prepared in an E. coli host derived from the BL21 strain. E. coli were grown to log-phase and infected with T7 phage and incubated with shaking at 32 °C until lysis. The lysates were centrifuged and filtered to remove Cells were detached using 0.05% trypsin-EDTA. Cultures were incubated at 37 ℃, 5% CO2. 96 well plates were seeded at 10,000 cells/well and incubated for 24 hours. Media was then exchanged for compound media (serial dilutions from 300 μM to 37 nM, or 100 μM to 25 nM) and incubated for 48 hours. Compound media was then exchanged for unmodified media, 10% alamar blue. Plates were then incubated for 4 hours and analysed using a Clariostar TM plate reader and the Flourescence Intensity (540-20 nM/590-30 nM) was measured. Each concentration was plated in triplicate, and 3 controls containing DMSO only, no alamar blue and no cells were also plated in triplicate. Values were normalised to give percentage inhibition using the no cells control as 100% inhibition and the DMSO control as 0% inhibition. Experiments were performed 3 times and all 9 wells were then grouped, and a nonlinear regression curve with four parameters was then plotted using GraphPad Prism, generating activity data.