GlaxoSmithKline plc (GSK) is a British pharmaceutical company headquartered in Brentford, London. Established in 2000 by a merger of Glaxo Wellcome and SmithKline Beecham, GSK was the world's sixth largest pharmaceutical company as of 2015, after Pfizer, Novartis, Merck, Hoffmann-La Roche and Sanofi. Andrew Witty has been the chief executive officer since 2008.