Tracing the dynamic distribution pattern of EGFP-VDR, EGFP-PPAR/nlsm and EGFP-RARα/nlsm (homogeneous in the absence of RXRα and nuclear-enriched in response to RXRα binding) serves as a good model system for studying their competition for heterodimerization with RXRα. There is indeed dynamic competition between RXR partners, which is governed by two mechanisms. First, in the absence of agonist treatment, there is a hierarchy of affinities between RXRα and its partners in the following order: RARα > PPARγ > VDR. Second, in the presence of agonist treatment, RXRα partner selection is shifted towards the liganded partner. Our results also show that RXR-NR heterodimerization and direct DNA binding are correlated events, and both are augmented by agonist treatment. These results may explain certain side effects of drugs targeting NRs. Competition for RXR could be responsible for the symptoms of vitamin D deficiency developed in a child upon receiving systemic retinoid treatment for ichthyosis188, or the antagonistic effect of co-administered vitamin A on serum calcium response to vitamin D treatment189. Our observations regarding these three RXR partners, consistently with metabolism regulation by two other RXR partners (LXR and PPARα) and similar previous findings on membrane-localized IL-2/15 receptors encourage us to generalize the concept that specific ligand binding may often govern competition between different partners of a promiscuous receptor. Our findings are a proof-of-concept of a hierarchy of affinities between NRs and their common partner, RXRα. These studies can be extended to a larger number of receptors to uncover the network of hierarchies, and follow-up studies will also be focusing on testing this concept in a broader and physiological context. It is worth to mention that our or similar studies may have some limitations: i) we cannot entirely exclude the presence of endogenous ligands for the receptors; and ii) our approach may not mimic all possible physiological conditions; iii) as for the limiting nature of RXR, antibody-based approaches are at best semiquantitative to determine endogenous NR concentrations; iv) finally, we used indirect measures to assess downstream gene expression events.