One of the challenges with small molecule binders displaying on-target activity with recombinant proteins, is to translate their utility to a cellular context. To demonstrate cell permeability and targeted engagement with our molecules, we developed a split-NanoLuc based Cellular target engagement assay (Cellular Thermal Shift Assay -CETSA) involving transient expression of HiBiT tagged SOCS2 to demonstrate the cellular engagement for our binders . One of the advantages of CETSA is that target engagement can be monitored without the need for development of a cell permeable tracer molecule. For our initial assays we utilised the permeabilised cell format CETSA to overcome any potential permeability barrier and demonstrate a dose-dependent shift in temperature of aggregation (Tagg) mediated by MN551(Figure ). To test the cellular activity of MN551, we treated HeLa cells with 50 μM MN551 for 20 hours and performed CETSA in a live-cell mode. We observed a small Tagg shift of 2 °C in live mode, which increased to 3-4 °C Tagg shift in the permeabilised format (Figure ). We postulate that this observation could likely be a result of low cellular permeability of MN551 owing to its negatively charged phosphate group.