Odour is sensory stimulation of the olfactory membrane of the nose by a group of molecules. Certain body odours are connected to human sexual attraction. Humans can make use of body odour subconsciously to identify whether a potential mate will pass on favourable traits to their offspring. Body odour may provide significant cues about the genetic quality, health and reproductive success of a potential mate. Body odour affects sexual attraction in a number of ways including through human biology, the menstrual cycle and fluctuating asymmetry. The olfactory membrane plays a role in smelling and subconsciously assessing another human's pheromones. It also affects the sexual attraction of insects and mammals. The major histocompatibility complex genes are important for the immune system, and appear to play a role in sexual attraction via body odour. Studies have shown that body odor is strongly connected with attraction in heterosexual females. The women in one study ranked body odor as more important for attraction than “looks”. Humans may not simply depend on visual and verbal senses to be attracted to a possible partner/mate. Olfactory epithelium The olfactory epithelium is a thick yellow/brown structure, about one inch square, located in the upper nasal cavity of the human nose. Made up of olfactory receptors and glands, the epithelium is used as a tool to smell others' body odor and pheromones. Chemicals that produce odour pass through the olfactory epithelium to the olfactory bulbs, which contain biological receptors that detect the chemicals, and respond with an electrical signal transmitted to the brain by the olfactory nerves. The olfactory epithelium plays a large role in why humans are attracted to persons biologically rather than physically; this relates directly to the sense of smell and not physical appearance. Olfactory communication is common in all animals and recent studies have shown that humans have this communication trait as well. This kind of communication happens subconsciously, and often influences a person's attraction to another. Pheromones Pheromones are chemical messengers produced and emitted by the body that contribute significantly to interpersonal attraction. The two types of pheromones include signal and primer, each playing a distinct role in human behavior. Signal pheromones act as attractants and repellents; they are classified as short term behavioral pheromones. Primer pheromones produce long term changes in human behavior and hormone production. The vomeronasal organ is used to detect the pheromones of others. Pheromones emitted from sweat glands play