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SARS coronavirus proteins. Computer artwork of SARS coronavirus particles (virions). SARS (severe acute respiratory syndrome) is a viral respiratory illness caused by the SARS-associated coronavirus (SARS-CoV); a single stranded RNA (ribonucleic acid) virus. Each SARS-CoV virion is made up of four major structural proteins: nucleocapsid, spike trimer (red), membrane, and envelope, plus HE haemagglutinin-esterase dimer (yellow). (KEYSTONE/SCIENCE PHOTO LIBRARY/RAMON ANDRADE 3DCIENCIA/SCIENCE PHOTO LIBRARY)