A plasmid found in the cytoplasm of certain bacteria that is responsible for initiating conjugation and gene transfer. The sex factor in Escherichia coli is known as the F factor; it can be transferred from a donor cell (F⁺) to a recipient (F⁻) by conjugation, enabled via formation of a sex pilus. Occasionally the F factor may become integrated with the chromosome of an F⁺ bacterium to form a high-frequency recombinant (Hfr) cell. This enables the part of the host chromosome nearest to the F factor to be transferred to the recipient by conjugation.