Retinoic acid receptor (RAR), thyroid hormone receptor (T3R) and vitamin D3 receptor (VD3R) differ from steroid hormone receptors in that they bind and transactivate through responsive elements organized as direct rather than inverted repeats. We now show that recombinant RAR and T3R are monomers in solution and cannot form stable homodimeric complexes on their responsive elements. Stable binding of the receptors to their responsive elements requires heterodimerization with a nuclear factor. This auxiliary factor is tightly associated with RAR and T3R in the absence of DNA and co-purifies with both receptors. As demonstrated by extensive purification, the same auxiliary factor is required for stable DNA binding of RAR as for that of T3R; the factor also facilitates the formation of a stable VD3R-DNA complex. The auxiliary factor is identical to the retinoid X receptor alpha (RXR alpha) by biochemical and functional criteria. The identification of RXR alpha as a dimerization partner for the RARs, T3Rs and VD3R has important implications as to the function of these receptors and their ligands in development, homeostasis and neoplasia.