Horizontal gene transfer is the movement of genetic information other than by the transmission of DNA from parent to offspring. This type of gene transfer is most prevalent among microorganisms. There are three main mechanisms of horizontal gene transfer between microorganisms: conjugation, transformation and transduction. Conjugation represents the transfer of mobile genetic elements, usually plasmids, through the sex pilus. Bacteria containing the F plasmid are called F+ bacteria and they are plasmid donors, converting F- bacteria into F+. The F plasmid can also be incorporated into the host's chromosome, resulting in Hfr strains. Transformation is the introduction of environmental DNA into the host. Bacteria capable of receiving environmental DNA are called competent bacteria. Transduction is phage-mediated horizontal gene transfer. There is a difference between specialized transduction, where only specific genes are transferred, and generalized transduction, where any gene can be transferred. Horizontal gene transfer enables microorganisms to quickly adapt to environmental conditions and is an important mediator of prokaryotic evolution. The best examples of this are proteorhodopsin and antibiotic resistance, both important adaptations that are widespread precisely because of the positive effect on the viability of the individuals that possess them.