ERVW-1 and infection: Thus, alterations in the transgene cassette of vector mutants were detected by PCR of genomic DNA extracted from infected cells at the end of each infection cycle (Fig. 3A) and from infected tumours 5, 10, and 20 days following vector inoculation, as previously described [35] (Fig. 3B), using primers annealing to sequences in the env gene and in the 3' LTR.