a role in sexual attraction, sexual repulsion, mother-infant bonding, and menstrual cycles. Biology There is a relationship between various aspects of human biology and genetics with sexual attraction. This includes the role of the major histocompatibility complex (MHC), the human leukocyte antigen (HLA) and their different heterozygotic variations. Such genetic factors may play a role in sexual selection. Signalling odours in reproduction are called attractants; their function is to bring about successful mating. The MHC (major histocompatibility complex) is a group of genes essential for the immune system, playing an important role in immunological recognition. These olfactory cues are involved in mate choice and preferences. HLA refers to the human form of MHC, and is a gene complex which encodes the MHC. MHC There is a correlation between mate choice, odour preference and genetic similarity at the MHC. Unique body smells are heavily influenced by MHC; these olfactory cues are probably involved in mate choice and preferences. MHC is expressed codominantly; a more diverse set of MHC genes leads to a stronger immune system. Women prefer male partners with differing MHC genes from themselves. An evolutionary explanation is that females are attracted to males with MHC alleles different from their own, to provide their offspring with a stronger immune system. Females not using hormonal contraceptives were more attracted to the scent of males with dissimilar MHCs. Females currently using hormonal contraceptives preferred the scent of men with MHCs similar to their own. The scent of an individual with low fluctuating asymmetry is universally more attractive. The increased attraction between people of dissimilar MHCs may help to prevent incest and subsequent birth defects. This inbreeding avoidance hypothesis proposes that biases towards heterozygotic alleles prevent harmful genetic consequences which can arise from mating with genetic family members. Olfactory cues can be used to recognise kin. Couples who have many failed attempts at conception share a considerably larger amount of genes than those who are able to conceive without problems. If, for these genetically similar couples, there is a successful conception, the babies are often born early or underweight. A third hypothesis explaining the function of dissimilar MHC mate preferences, the parasite hypothesis, suggests that MHC heterozygotes may be resistant to rapidly evolving parasites. HLA The human leukocyte antigen system is a protein complex which is encoded by MHC in humans. Over thirty olfactory receptor genes have been located at the HLA class