SUMMARY: For metabolite annotation in metabolomics, variations in the registered 
states of compounds (charged molecules and multiple components, such as salts) 
and their redundancy among compound databases could be the cause of 
misannotations and hamper immediate recognition of the uniqueness of metabolites 
while searching by mass values measured using mass spectrometry. We developed a 
search system named UC2 (Unique Connectivity of Uncharged Compounds), where 
compounds are tentatively neutralized into uncharged states and stored on the 
basis of their unique connectivity of atoms after removing their stereochemical 
information using the first block in the hash of the IUPAC International 
Chemical Identifier, by which false-positive hits are remarkably reduced, both 
charged and uncharged compounds are properly searched in a single query and 
records having a unique connectivity are compiled in a single search result.
AVAILABILITY AND IMPLEMENTATION: The UC2 search tool is available free of charge 
as a REST web service (http://webs2.kazusa.or.jp/mfsearcher) and a Java-based 
GUI tool.
CONTACT: sakurai@kazusa.or.jp.
SUPPLEMENTARY INFORMATION: Supplementary data are available at Bioinformatics 
online.