Our results indicate that replication/transcription of a genome segment can be negatively affected by the presence of another genome segment. This interference became less pronounced when the length of the smaller segment was extended, indicating that genome segment length plays a role in the competition between different segments. This ''length effect'' was also observed when natural genome segments were present in addition to the reporter construct, with the shortest segments, M and NS, giving the strongest inhibition of the reporter gene expression. In agreement herewith, IAV defective interfering (DI) RNAs, which are formed by internal deletion of progenitor RNA segments, interfere with vRNA synthesis, probably because of the competitive advantage of the smaller DI RNA molecules (reviewed by Nayak [29] ). In addition, our data indicate that the coding region of the vRNA segment may also be of importance, as the extended version of the Gaussia reporter segment was still able to outcompete its firefly luciferase counterpart, albeit less efficiently than its shorter version.