Horizontal gene transfer is any process in which an organism transfers genetic material to another cell that is not its offspring. Vertical transfer occurs when an organism receives genetic material from its ancestor such as from its parent or a species from which it evolved.

Horizontal gene transfer is common among bacteria, even very distantly related ones. For example, this process is thought to be a significant cause of increased drug resistance; when one bacterial cell acquires resistance, it can quickly transfer the resistance genes to many species. Also enteric bacteria appear to exchange genetic material with each other within the gut in which they live.