According to previous studies of SARS-CoV (Severe acute respiratory syndrome coronavirus), a variety of novel accessory genes have been identified in SARS-CoV genome, which were interspersed the structural genes of SARS-CoV and considered to be unique to the SARS-CoV genome. The predicted unknown proteins (PUPs) encoded by the accessory genes might play important roles in the SARS-CoV infection. Three of those genes, called X4, X5 and ORF10, were synthesized and introduced into E. coli to induce expression. SDS-PAGE and Western blotting revealed that the three genes have been expressed in E. coli. The induction of SARS PUPs genes expression in different temperatures, induction times, IPTG concentrations and A values of E. coli cells were performed. The optimal induction condition of SARS-CoV PUPs genes was characterized according to the orthorgonal analysis. The ratio of recombinant proteins of PUPs to total proteins is as follows: X4, 20%; X5, 27.8%; ORF10, 68.5% under the optimum conditions.