The Science Citation Index (SCI) is a citation index originally produced by the Institute for Scientific Information (ISI) in 1960, which is now owned by Thomson Reuters. The larger version (Science Citation Index Expanded) covers 6,400 of the world's leading journals of science and technology. It is made available online through the Web of Science database, a part of the Web of Knowledge collection of databases. (There are also CD and printed editions, covering a smaller number of journals). This database allows a researcher to identify which later articles have cited any particular earlier article, or cited the articles of any particular author, or determine which articles have been cited most frequently. Thomson Reuters also market several subsets of this database, termed "Specialty Citation Indexes", such as the Neuroscience Citation Index.