Abstract
A model for F-prime formation is presented. It predicts that an Hfr strain giving rise to an F-prime factor would acquire a deletion corresponding to the chromosomal fragment carried by the episome. Genetic studies have confirmed this prediction. Concomitant transfer to the episome of a gene determining a function vital to the cell has permitted selection of derived Hfr strains in which the episomal fragment has been translocated to various sites on the bacterial chromosome.