AbstractMarek’s disease virus (MDV) is an alphaherpesvirus that causes fatal lymphomas in chickens and is used as a natural virus-host model for herpesvirus-induced tumorigenesis. MDV encodes a telomerase RNA subunit (vTR) that is crucial for efficient MDV-induced lymphoma formation; however, the mechanism is not completely understood. Similarly, Epstein Barr-virus (EBV) encodes two RNAs (EBER-1 and EBER-2) that are highly expressed in EBV-induced tumor cells, however their role in tumorigenesis remains unclear. Intriguingly, vTR and EBER-1 have interaction partners in common that are highly conserved in humans and chickens. Therefore, we investigated if EBER-1 and/or EBER-2 can complement the loss of vTR in MDV-induced tumor formation. We first deleted vTR (v∆vTR) and replaced it by either EBER-1 or EBER-2 in the very virulent RB-1B strain. Insertion of either EBER-1 or EBER-2 did not affect MDV replication and their expression levels were comparable to vTR in wild type virus. Intriguingly, EBER-2 restored tumor formation of MDV that lacks vTR. EBER-1 partially restored MDV oncogenicity, while tumor formation was severely impaired in chickens infected with v∆vTR. Our data provides the first evidence that EBERs possess tumor-promoting properties in vivo using this natural model for herpesvirus-tumorigenesis.