Expression of the GP5 protein of porcine reproductive and respiratory syndrome virus in mammalian cells using a recombinant vaccinia virus has been shown to induce strong cytotoxicity due to apoptotic death. We have now developed a transient expression system that allows the observation and quantitation of the cell death due to GP5 synthesis, taking advantage of the reduction that this protein induces in the expression of two different co-transfected reporter genes. In this way, we are able to study the regions in GP5 implicated in apoptosis induction. The first 119 aminoacids constitute a region capable of fully inducing apoptosis, aminoacids 90-119 having a fundamental role. On the contrary, the C-terminal region is unable by itself of cell death induction and, moreover, is dispensable for this phenotype. We have also observed that induction of apoptosis is independent of cleavage of the N-terminal putative signal sequence in GP5 or N-glycosylation of this protein.