X-ray structure of the homodimeric glucocorticoid receptor (GR) DNA-binding domain in complex with an 18-bp dsDNA. The DNA contains two inverted repeats of the 6-bp glucocorticoid response element (GRE) half-sites (5′-AGAACA-3′) separated by a 4-bp spacer (GRE_4S). The complex is initially viewed with its approximate 2-fold molecular axis horizontal. The protein is initially shown in ribbon form with the lower subunit in the initial view colored in rainbow order from its N-terminus (blue) to its C-terminus (red), the upper subunit pink, and its bound Zn²⁺ ions represented by cyan spheres. The DNA is drawn in stick form with the C atoms of its two 6-bp GRE half-sites magenta, the C atoms of the remaining nucleotides green, N blue, O red, and P orange and with successive P atoms connected by orange rods. Note that the GR's N-terminal helices are inserted into adjacent major grooves of the DNA. Click on the "Show DNA–Helix Interactions" button for a detailed view of how the lower subunit's N-terminal helix is inserted into the major groove. Only the lower subunit binds to the DNA in a sequence-specific manner; the upper subunit binds to the palindromic DNA one base pair closer to the center of the DNA molecule than does the lower subunit and hence does not make sequence-specific contacts with the DNA. This can be seen by turning the complex by 180° about the horizontal (x) axis. [Based on an X-ray structure by Paul Sigler, Yale University. PDBid 1GLU.]