A novel coronavirus has recently been identified as the causative agent of SARS (Severe Acute Respiratory Syndrome). Coronaviruses are a major cause of upper respiratory diseases in humans. The genomes of these viruses are positive-stranded RNA approximately 27-31kb in length. SARS infection can be mediated by the binding of the viral spike protein, a glycosylated 139 kDa protein and the major surface antigen of the virus, to the angiotensin-converting enzyme 2 (ACE2) on target cells. This binding can be blocked by a soluble form of ACE2.