Mice bearing this transgene serve as a tool for assessing the mutagenicity of environmental chemicals. The vector can be rescued from mouse tissues using an in-vitro packaging system and used to infect E. coli. Long deletions inactivating both red and gam permit plaque formation on P2-lysogenic E. coli (the spi- phenotype). Upon infection of a Cre-expressing E. coli strain, the plasmid is linearized, circularized and maintained at a high copy number. Bacterial colonies bearing mutations of gpt can be identified by growth on media containing chloramphenicol and 6-thioguanine; point mutations and short deletions of the 456 base pair gpt gene are easily characterized by sequence analysis.