The NCBI Related Structures tool allows you to find 3D structures from the Molecular Modeling Database (MMDB) that are similar in sequence to a query protein.

This server allows to search CATH using a PDB structure.

Search the Molecular Modeling Database (MMDB) at NCBI

OCA is a browser-database for protein structure/function.

ProteinDBS is the first real-time search engine for retrieving similar protein tertiary structures in Protein Data Bank (PDB). This system provides two types of query method: query by PDB protein chain ID and by 3D coordinates of newly-discovered proteins in PDB format. With computational techniques, similar protein structures with ranked order will be returned to users in seconds. Our database proteins are weekly updated to be consistent with PDB database.

VAST Search is NCBI's structure-structure similarity search service. It compares 3D coordinates of a newly determined protein structure to those in the MMDB/PDB database. VAST Search computes a list of structure neighbors that you may browse interactively, viewing superpositions and alignments by molecular graphics.

TOPSCAN is a rapid method for comparing protein structures. It is useful as a preliminary screen to give you some idea whether your protein looks like something else.

World Index of Molecular Visualization Resources for PDB Files

Protein Quaternary Structure Query Form at the EBI

PubChem provides information on the biological activities of small molecules.

Search in 74 million indexed structures from over 100 databases (46 million unique structures).