We observed DNA laddering patterns for H. ericina and P. pouchetii cultures upon treatment with camptothecin (Supplementary data,Fig. S1), a DNA topoisomerase I inhibitor known to induce replication arrest and PCD (Sen et al., 2004), thereby confirming the inducible PCD-like DNA fragmentation phenotype associated with virus infection. This evidence concerns the gene TOP1 and viral infectious disease.