Pathogenicity islands are discrete genetic elements that encode virulence factors, such as toxins, adhesins, secretion systems, and iron transport proteins. These islands, which range in size from 10 to 200 kB, can be horizontally transferred between bacteria, resulting in enhanced virulence and fitness in the recipient. [Note: Horizontal gene transfer is any process (such as transformation, transduction, or bacterial conjugation) in which an organism incorporates genetic material from another organism without being the progeny of that organism. By contrast, vertical transfer occurs when an organism receives genetic material from its ancestor, for example, a species from which it has evolved.] Pathogenicity islands differ from the rest of the chromosome in G+C content and are usually flanked in the recipient's chromosome by repeated sequence elements or genes that encode tRNAs.