An Hfr cell (also called an Hfr strain) is a bacterium with a conjugative plasmid (often the F-factor) integrated into its genomic DNA. Hfr is the abbreviation for high frequency recombination, which was first characterized by Luca Cavalli-Sforza. Unlike a normal F+ cell, hfr strains will, upon conjugation with a F− cell, attempt to transfer their entire DNA through the mating bridge, not to be confused with the pilus. This occurs because the F factor has integrated itself via an insertion point in the bacterial chromosome. Due to the F factor's inherent nature to transfer itself during conjugation, the rest of the bacterial genome is dragged along with it, thus making such cells very useful and interesting in terms of studying gene linkage and recombination. Because the genome's rate of transfer through the mating bridge is constant, molecular biologists and geneticists can use Hfr strain of bacteria (often E. coli) to study genetic linkage and map the chromosome. The procedure commonly used for this is called interrupted mating.

A bacterium may undergo conjugation. During this process, genetic material is transferred to another bacterium through the mating bridge. It is yet undetermined if the Pili transfer DNA or if these structures are simply used to bring mating bacteria close enough to form a mating bridge.[1] To form pili, an F plasmid is required. The F plasmid consists of 28 genes which are mostly required for the production of the pilus. F+ denotes cells that contain the F plasmid, while F− cells do not. The F plasmid is considered to be an episome which may become integrated into the main chromosome. When the F genes become integrated into the chromosome, the cell is said to be Hfr (high frequency of recombination). An Hfr cell may transfer F genes to an F− cell. During this transfer of genetic material, the F episome may take chromosomal DNA with it. The donor cell does not lose any genetic material as anything transferred is replicated concurrently. It is extremely rare that an Hfr cell's chromosome is transferred in its entirety. Homologous recombination occurs when the newly acquired DNA crosses over with the homologous region of its own chromosome.

A structure as fragile as a mating bridge will, however, likely break, and so the transfer is rarely complete. Thus, the F− cell uses only part of the genomic DNA of the Hfr cell for recombination. Though there is some debate on the issue, the pili themselves are not the structures through which the actual exchange of DNA takes place; rather, a Type IV secretion system is used to transfer DNA between the bacteria.