The tumor virus A (Tva) sequence disrupted with a lox-stop-lox cassette after the initiation codon, an IRES element and lacZ were inserted into intron 1 via homologous recombination. Cre mediated recombination results in expression of TVA and lacZ. Expression of TVA makes cells vulnerable to infection by RCAS (replication-competent sarcoma-leukosis virus long terminal repeat with splice acceptor) vector with Bryan-RSV polymerase and subgroup A envelope viruses (RCASBP(A)). (J:140423)