As a result, the exclusion process must stochastically select a single viral RNA per cell, resulting in a ��salt and pepper�� pattern across a population of cells. We show that a replicon lacking ORF1 cannot be selected in this stochastic process, and that individual replicase molecules prefer the RNA that encoded them. Only on rare occasions does the RdRP accept the ��foreign�� replicon RNA as a template, resulting in low levels of co-expression. As a consequence, the number of cells expressing Irbesartan defective helper transgenes, which crucially depend on trans-activation, was low. Combining our data with the previous studies of superinfection exclusion reveals important new aspects of this model. First, the presence of two RdRP-bearing viral replicons appears to lead to a competition in which only one RdRP molecule becomes stochastically ����licensed���� to be active. As previously suggested, we propose that the first polyprotein molecule to be translated immediately begins the exclusion process by proteolytic inactivation of all other replicase molecules in the cell. Second, this active RdRP strongly prefers to transcribe the antigenome to the RNA that it originated from, presumably through close physical contact in a cytoplasmatic protein/RNA complex. Using point-mutated replicons and deficient helpers, we show that at the same low frequency, either a second RdRP gets licensed, or a single RdRP can replicate a second viral mRNA. As a result of these two mechanisms, we conclude that only one RNA molecule per cell is typically selected for replication. We note that this model is also entirely consistent with our observations, and those of others, that point-mutated replicons can mediate exclusion, as SinRGFP still produces an important cleavage product, the protease nsp2. Moreover, these studies provide genetic support for the observed isolation of replication complexes within membrane bound intracellular organelles, as it is possible that the single, licensed genome could occupy such vesicular structures. While we favor this model, we cannot exclude specific variations of alternative models that have been proposed for other viruses, including a possible competition for host factors, or the Primidone involvement of virus proteins.