The recent emergence of two highly pathogenic human coronaviruses (Co. Vs), severe acute respiratory syndrome CoV and Middle East respiratory syndrome CoV, has ignited a strong interest in the identification of viral factors that determine the virulence and pathogenesis of Co. Vs. The nonstructural protein 1 (nsp1) of Co. Vs has attracted considerable attention in this regard as a potential virulence factor and a target for CoV vaccine development because of accumulating evidence that point to its role in the downregulation of host innate immune responses to CoV infection. Studies have revealed both functional conservation and mechanistic divergence among the nsp1 of different mammalian Co. Vs in perturbing host gene expression and antiviral responses. This review summarizes the current knowledge about the biological functions of CoV nsp1 that provides an insight into the novel strategies utilized by this viral protein to modulate host and viral gene expression during CoV infection.