An assay system for the pin gene function, which suppresses the vh2 mutation of Salmonella, was developed and used to show that most strains of Escherichia coli K-12 are Pin⁺, whereas all the strains of E. coli C examined are Pin⁻. An E. coli host strain was constructed and used for detection of DNA fragments carrying the E. coli K-12 pin gene cloned in the plasmid vector pBR322. Restriction analysis of the cloned fragments showed that the invertible DNA (designated P region) is adjacent to the pin gene and that its inversion is mediated by the pin gene product. The pin gene was found to be functionally homologous to the gin gene of Mu phage and the cin gene of P1 phage. The P̄ region most probably resides within the cryptic prophage e14, and the Pin⁻ phenotype is likely to be associated with the loss of e14.