LOOPP is a public domain package for protein recognition and designing folding potentials. LOOPP includes standard and novel (developed by myself and Ron Elber) sequence to structure (threading), sequence to sequence and structure to structure alignment algorithms and potentials. LOOPP can also be used to develop new folding potentials. Previously known as Linear Optimization Of Protein Potentials. To download the source code of the new version (LOOPP 2000) follow the links included below or go to the official LOOPP web page at the Cornell Theory Center.