Nowadays, different sexualities and gender identities are rapidly becoming more and more accepted in mainstream society. Despite this change, many people believe that having a different sexual orientation or gender identity is a frowned upon choice. To disprove this belief, research into brain biology is needed. The study of the brain on the basis of sexuality is a relatively new topic of discussion as it is somewhat difficult and confusing. This article will explore the different gender, sexual and sexual orientation identities and the main biological reasons behind them. There is also some validity to different sexual orientations and identities through evidence of sexual disorders like Klinefelter-Turner syndrome and gender dysphoria. In order to discuss the biology of gender identity and sexual orientation, it is necessary to first examine the differences between several definitions. which are often wrongly interchanged: sex, gender, sexual orientation and gender identity. Sexual orientation is defined by LeVay (2011) as “the trait that predisposes us to experience sexual attraction toward people of the same sex, toward people of the other sex, or toward both sexes” (p. 1 ). Typical sexual orientation categories are homosexual, heterosexual, and bisexual. Vrangalova and Savin-Williams (2012) found that most people identify as heterosexual, but there are also groups of people who identify as mostly heterosexual and mostly gay within the three traditional categories (e.g. 89). This means that there are not three concrete groups, but sexual orientation is a continuum and we can even fluctuate on it over time. LeVay (2011) also defines gender as "the...... middle of article...... published October 9, 2015 at http://www.mayoclinic.com/health/klinefelter-syndrome/ DS01057.Swaab, D.F. and Garcia-Falgueras, A. (2009). Sexual differentiation of the human brain in relation to gender identity and sexual orientation. Functional Neurology, 24(1), 17-28. Turner Syndrome Society. (2011). Learn more about TS: fact sheet. Turner Syndrome Society of the United States. Retrieved October 9, 2015 from http://turnersyndrome.org/learn-about-ts/fact-sheet Vrangalova, Z. and Savin-Williams, RC (2012). Mostly heterosexual and mostly gay/lesbian: evidence of new sexual orientation identities. Archives of Sexual Behavior, 41, 85-101. doi:10.1007/s10505-012-9921-y.Zhang, S., Liu, Y., and Rao, Y. (2013). Serotonin signaling in the brains of adult female mice is necessary for sexual preference. Proceedings of the National Academy of Sciences, 110(24), 9968-9973.