Severe acute respiratory syndrome (SARS) coronavirus (SARS-CoV) caused a severe outbreak in several regions of the world in 2003. The virus is a novel coronavirus isolated from patients exhibiting atypical pneumonia and may have originated from wild animals such as civet cats in southern China. The genome of SARS-CoV is a positive-sense, single-stranded RNA whose sequence is distantly related to all known coronaviruses that infect humans and animals. Like other known coronaviruses, SARS-CoV is an enveloped virus containing three outer structural proteins, namely the membrane (M), envelope (E), and spike (S) proteins. The nucleocapsid (N) protein together with the viral RNA genome presumably form a helical core located within the viral envelope. The SARS-CoV nucleocapsid (N) protein is a 423 amino-acid, predicted phospho-protein of 46 kDa that shares little homology with other members of the coronavirus family. A short serine-rich stretch, and a putative bipartite nuclear localization signal are unique to it, thus suggesting its involvement in many important functions during the viral life cycle. In this report we have cloned the N gene of the SARS coronavirus, and studied its property of self-association to form dimers. We expressed the N protein as a fusion protein in the yeast two-hybrid system to demonstrate self-association and confirmed dimerization of the N protein from mammalian cell lysates by coimmunoprecipitation. Furthermore, via deletion analysis, we have shown that the C-terminal 209 amino-acid region constitutes the interaction domain responsible for self-association of the N protein to form dimers.