The Molecular Modelling Database (MMDB) contains 3-dimensional structures determined by X-ray crystallography and NMR spectroscopy. The data is obtained from the Protein Data Bank (PDB). NCBI has cross-linked structural data to bibliographic information, sequence databases, and the NCBI taxonomy. Cn3D, a 3D structure viewer, provides interactive visualization of molecular structures from within Entrez. MMDB protein structures have been compared to each other with VAST (Vector Alignment Search Tool).