An in vitro ubiquitination assay was employed for an initial assessment of the ability for ISOX DUALbased degraders to induce productive ternary complex formation. This assay follows ubiquitination of the target proteins in a cell-free system, removing potentially confounding factors such as Degrader cell permeability and efflux. We first tested a subset of VHL-recruiting Degraders (compounds 42 -45)