Ff phages have been engineered for applications in biological and medical sciences. Many applications build on experiments showing that the DNA sequence determining resistance to the antibiotic kanamycin can be inserted in a functional form into the non-coding intergenic sequence of fd phage DNA. Such modified phage are correspondingly longer that wild-type filamentous fd, because the longer DNA is coated with correspondingly more gene 8 coat proteins, but the phage life-cycle is not otherwise disrupted. The traditional “tadpole” or isometric shaped-phage, on the other hand, which have a limited-sized capsid, cannot be so easily used to encapsidate a larger DNA molecule. The modified phage can be selected by infecting kanamycin-sensitive bacteria with modified phage to introduce resistance to kanamycin, and growing the infected bacteria in media containing an otherwise lethal concentration of kanamycin.
This result was extended by inserting foreign DNA expressing a foreign peptide into fd phage gene 3, rather than  into the intergenic sequence, so that the foreign peptide appears on the surface of the phage as a part of the gene 3 adsorption protein. Phage carrying  the foreign peptide can then be detected  using appropriate antibodies. The reverse of this approach is to insert DNA coding for antibodies into gene 3 and detect their presence by appropriate antigens.
These techniques have been extended over the years in many ways, for instance by inserting foreign DNA into the genes coding for phage coat proteins other than gene 3, and/or duplicating the gene of interest to modify only some of the corresponding gene products. Phage display technology has been widely used for many purposes.