Vectored vaccines based on highly attenuated modified vaccinia Ankara (MVA) are reported to be immunogenic,tolerant to pre-existing immunity,and able to accommodate and stably maintain very large transgenes.MVA is usually produced on primary chicken embryo fibroblasts,but production processes based on continuous cell lines emerge as increasingly robust and cost-effective alternatives.An isolate of a hitherto undescribed genotype was recovered by passage of a non-plaque-purified preparation of MVA in a continuous anatine suspension cell line (CR.pIX) in chemically defined medium.The novel isolate (MVA-CR19) replicated to higher infectious titers in the extracellular volume of suspension cultures and induced fewer syncytia in adherent cultures.We now extend previous studies with the investigation of the point mutations in structural genes of MVA-CR19 and describe an additional point mutation in a regulatory gene.We furthermore map and discuss an extensive rearrangement of the left telomer of MVA-CR19 that appears to have occurred by duplication of the right telomer.This event caused deletions and duplications of genes that may modulate immunologic properties of MVA-CR19 as a vaccine vector.Our characterizations also highlight the exceptional genetic stability of plaque-purified MVA:although the phenotype of MVA-CR19 appears to be advantageous for replication,we found that all genetic markers that differentiate wildtype and MVA-CR19 are stably maintained in passages of recombinant viruses based on either wildtype or MVA-CR.