Nuclear hormone receptor responsive element binding specificity has been reported to reside predominantly in the proximal box (P-box), three amino acids located in a DNA-recognition -helix situated on the C-terminal side of the first zinc finger. To further define the residues in the vitamin D receptor (VDR) DNA binding domain (DBD) that mediate its interaction as a retinoid X receptor (RXR) heterodimer with the rat osteocalcin (rOC) vitamin D responsive element (VDRE), chimeric receptors were created in which the core DBD of VDR was replaced with that of the homodimerizing glucocorticoid receptor (GR). Systematic alteration of GR DBD amino acids in these chimeras to VDR DBD residues identified arg-49 and lys-53, just C-terminal of the P-box within the base recognition -helix of human VDR (hVDR), as the only two amino acids among 36 differences required to convert the GR core zinc finger domain to that of the VDR. Gel mobility shift and 1,25-dihydroxyvitamin D₃-stimulated transcription assays verified that an hVDR-GR DBD chimera is functional on the rOC VDRE with only the conservative change of lys-49 to arg, and of the negatively charged glu-53 to a basic amino acid (lys or arg). Thus, for RXR heterodimerizing receptors like VDR, the P-box requires redefinition and expansion to include a specificity element (S-box) corresponding to arg-49 and lys-53 of hVDR. Examination of S-box amino acids in other nuclear receptors in terms of conservation and base contact in co-crystal structures supports the conclusion that these residues are crucial for selective DNA recognition.