Relibase is a web-based system for searching and analyzing protein-ligand structures in the Protein Data Bank (PDB). Data mining is the first step of every structural biology project. Relibase provides an easily accessible web-browser interface and clear 3D structure visualization. Use Relibase to explore structures using text, 2D substructure, SMILES and SMARTS searches. 3D protein-ligand interaction searches can be easily performed using the integrated sketcher allowing you to understand and assess the interactions that are important to molecular recognition in protein-ligand systems. Sophisticated automatic superimposition and detailed analysis of related binding sites allow you to identify protein flexibility, ligand overlap, and conserved water positions. Relibase features comprehensive coverage of the PDB and is regularly updates with newly deposited structures. Each structure uploaded to Relibase undergoes extensive validation and processing including the accurate assignment of ligand atom and bond types, generation of ligand diagrams and comprehensive secondary structure assignments.