to infant socialization. Each person produces a unique spread of pheromones that can be identified by others. This differentiation allows the formation of sexual attraction and kinship ties to occur. Sebaceous and apocrine glands become active at puberty. This, as well as many apocrine glands being close to the sex organs, points to a role related to mating. Sebaceous glands line the human skin while apocrine glands are located around body hairs. Compared to other primates, humans have extensive axillary hair and have many odor producing sources, in particular many apocrine glands. In humans, the apocrine glands have the ability to secrete pheromones. These steroid compounds are produced within the peroxisomes of the apocrine glands by enzymes such as mevalonate kinases. Sexual selection Pheromones are a factor seen in the mating selection and reproduction in humans. In women, the sense of olfaction is strongest around the time of ovulation, significantly stronger than during other phases of the menstrual cycle and also stronger than the sense in males. Pheromones can be used to deliver information about the major histocompatibility complex (MHC). The MCH in humans is referred to as the Human Leukocyte Antigen (HLA). Each type has a unique scent profile that can be utilized during the mating selection process. When selecting mates, women tend to be attracted to those that have different HLA-types than their own. This is thought to increase the strength of the family unit and increase the chances of survival for potential offspring. Studies have suggested that people might be using odor cues associated with the immune system to select mates. Using a brain-imaging technique, Swedish researchers have shown that homosexual and heterosexual males' brains respond in different ways to two odors that may be involved in sexual arousal, and that homosexual men respond in the same way as heterosexual women, though it could not be determined whether this was cause or effect. When the study was expanded to include lesbian women, the results were consistent with previous findings – meaning that lesbian women were not as responsive to male-identified odors, while responding to female odors in a similar way as heterosexual males. According to the researchers, this research suggests a possible role for human pheromones in the biological basis of sexual orientation. Kinship communication Humans can olfactorily detect blood-related kin. Mothers can identify by body odor their biological children, but not their stepchildren. Preadolescent children can