Pymol diagrams showing binding interactions between the NH2-terminal ATP-binding site of human HSP90α and the 3,4-diarylpyrazole resorcinol VER-49009 (upper) and 3,4-diarylisoxazole resorcinol VER-50589 (middle). Hydrogen bonds (dotted blue lines), amino acid residues involved (green), water molecules (cyan-coloured spheres) and residues in van der Waals contact (cyan). Also shown (lower) are orthogonal views of the bound structures of VER-49009 (cyan), VER-50589 (yellow) and ADP (green), overlaid by superimposition of the HSP90 NH2-domains from the crystal structures of the individual complexes. Protein X-ray crystallography was determined at 2.1 and 2.0 resolution for VER-49009 and VER-50589, respectively. VER-50589 exhibits the tightest binding of any synthetic small molecule yet reported, shows proof of concept for target inhibition and antitumor activity in an animal model, and illustrates the therapeutic potential of this new series of HSP90 inhibitors. For details, see Sharp et al. in this issue.