As the top two degraders from the second generation, XL01126 and XL01134 are epimers of each other, the only difference being swapped chirality at one of the two tertiary carbons of the cyclohexyl ring in their linkers. This small difference in chemical structure gives rise to very different degradation profiles for the two compounds (Figure and Figure ). These two epimeric PROTACs also exhibited strikingly different binding affinities to VHL as revealed by a fluorescence polarization (FP) displacement binding assay (Figure ) and a VHL target engagement assay (Figure ) . XL01126 has >10-fold weaker binary binding to VHL than XL01134 and also was found to be the weakest LRRK2 binder amongst the compounds tested (Figure ). PROTACs have previously been shown to tolerate weakened binary binding affinities to either their E3 ligase or target protein such that, despite the weak binding, they are able to induce potent protein degradation at a concentration well below the weakened Kd. Conversely, PROTACs made of more potent target ligands do not necessarily guarantee for more potent degraders . These studies together illustrated a now well-established feature with PROTACs, that is the extent of target degradation does not necessarily correlate with the PROTAC's binary binding affinity to E3 ligase or target protein. The ternary binding affinity, cooperativity and stability of the ternary complex, can instead play critically important roles in PROTAC induced protein degradation . To test whether our PROTACs can induce cooperative ternary complex formation and illuminate the relationship between the degradation potency and ternary complex formation, a ternary binding affinity assay and a ternary complex formation assay are warranted. However, we could not implement the mostly commonly used biophysical techniques such as fluorescence polarization and surface plasma resonance for these assays, due to the lack of sufficient recombinant expressed LRRK2 in hand. We therefore turned to endogenously expressed LRRK2 and developed a NanoBRET-based ternary binding affinity assay and ternary complex formation assay in HEK293 cells (Figure ).