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SARS coronavirus proteins. Cutaway computer model showing the protein structure of a SARS coronavirus particle (virion). SARS (severe acute respiratory syndrome) is a viral respiratory illness caused by the SARS-associated coronavirus (SARS-CoV); a single stranded RNA (ribonucleic acid, blue coil) virus. Each SARS-CoV virion is made up of four major structural proteins: nucleocapsid, spike trimer (red), membrane (yellow) and envelope (purple), plus HE haemagglutinin-esterase dimer (orange). (KEYSTONE/SCIENCE PHOTO LIBRARY/RAMON ANDRADE 3DCIENCIA/SCIENCE PHOTO LIBRARY)