Background: The mechanisms underlying chromosome instability (CIN) during cancer development are poorly understood.Recent data have shown that genomic instability can be triggered by cell fusion, which is often caused by viruses.The aim of the present study is to address the role of telomere shortening and mitotic instability in tumorigenesis related to virus-induced cell fusion.Result: Fibroblasts with cotransduction of two oncogenes, adenoviral E1 A and HRAS 1 were further infected with a retrovirus MPMVE, which gave rise to fused cells with transformed properties, including transplantable tumors in animals.Cytogenetic analysis of these cell subpopulations, viz., fused cell lineages and xenograft cell lineages derived from fusion cell, showed highly complex karyotypes with massive numerical as well as structural aberrations.Two mitotic events related to chromosome instability, i.e., anaphase bridging (AB) and mitotic multipolarity (MM) visualized by immunofluorescence for beta-tubulin and Aurora kinase A were evaluated in these virally transformed cell lines and the derived tumours.Both abnormalities were found in less than 1% of the metaphase/anaphase cells in control cells.In cells transduced by ElA and HRAS without cell fusion, AB and MM were found in 10% and 5 % of cells analyzed, respectively.In fused cell subpopulations, frequency of AB increased to 42-30%, whereas the frequency of AB decreased in tumor xenografts derived from the fused cells, to the range of 12-17%.The frequency of MM was similar in transduced cells, fused cells, and in xenograft tumors.Telomere length quantification analysis revealed that telomere length was significantly shortened in all virally fused cell subpopulations compared to the normal fibroblasts.Significance: Telomere dependant anaphase bridging, possibly in conjunction with disturbances of cell cycle checkpoints and premature chromosome condensation can be a critical mechanism for triggering chromosomal aberrations in tumorigenesis induced by viral cell fusion.