These results highlight a crucial step of assessing that the spy molecule solely binds to one the two proteins that the PROTAC recruits. F NMR competition experiments rely on fastexchanging binders with weak to intermediate affinity to act as effective spy molecules. To enable these features, suitable spy molecules are typically not as heavily functionalised as, for example, high-affinity tracers used in FP assays. Consequently, there is a higher probability that these fluorinated compounds might unexpectedly interact with undesired proteins. It should be noted that the effect of Brd4(2) in the F transverse relaxation of spy molecule 19 was still negligible (Figure ), indicating that no meaningful interaction between the bromodomain and this particular spy molecule was observed in the conditions at which the data shown in Table were measured. Nonetheless, the effect was sufficient interference, so we reverted back to spy molecule 19 in subsequent work. Graphs and fitting statistics can be found in the ESI (Figure and Table ).