and SMRT with the TLX LBD confirmed this assumption indicating similar or even higher affinity compared to the Atro box peptide (Figure and). The TLX-NCoR1 interaction showed no response to TLX modulators 8, 29, 30 or ccrp2 (Figure ), however, the interaction of TLX with the repressor SMRT was markedly decreased by istradefylline (30, Figure ) aligning with the inverse TLX agonism we observed in the VP16/TLX assay. These results suggest that TLX modulation by xanthines is at least in part mediated through the TLX-SMRT interaction. Still, additional mechanisms might involve in TLX modulation by ligands, and we hypothesized heterodimerization of TLX as potential further mediator of ligand effects in line with the repressor activity of TLX. When we titrated Tb 3+ -cryptate-labeled TLX LBD with sGFP-labeled RXRα LBD in presence of TLX ligands, we observed indeed enhanced dimerization for istradefylline (30) and ccrp2 while caffeine (8) and 29 had only weak effects (Figure ).