A typical P+ cell has 2-3 pili. Genes present on a plasmid called as P+ plasmid or F-factor control F -pili synthesis. This F+ plasmid is 94.5 kb is length with more than 60 known coding genes. This P+ plasmid can exist at a level of one copy per bacteria in free form and replicates as P+ cell.

But sometimes F+ plasmid gets integrated into the bacterial chromosome, ceases to exist in free form and replicates only when bacterial chromosome replicates. An interesting thing is, when P+ plasmid integrates into the host chromosome, it loses its ability to transfer F+ plasmid genes and instead transfers the host chromosomal genes.

Such mutant F+ strains are called as Hfr or high frequency recombinant strain or cell. The F+ can integrate into the host chromosome at anyone of many sites. The integration of the F factor is believed to be mediated by short DNA sequences called IS elements.