Arch Sex Behav (2007) 36:135–137 DOI 10.1007/s10508-006-9156-x LETTER TO THE EDITOR Monozygotic Twins Concordant for Female-to-Male Transsexualism: A Case Report Hans Knoblauch · Andreas Busjahn · Bernhard Wegener Published online: 18 January 2007 C  Springer Science+Business Media, LLC 2006 Transsexualism is a rare phenotype with an estimated fre- quency varying between 1 in 10,000 up to 1 in 100,000 in male-to-female transsexualism (MTF) and 1 in 30,000 up to 1 in 400,000 in female-to-male transsexualism (FTM) (Cohen- Kettenis & Gooren, 1999). Various explanations have been suggested ranging from biological to psychosocial factors (Coates, 1990; Cohen-Kettenis & Gooren, 1999; Zucker & Bradley, 1995). Emerging evidence supports brain morpho- logical, physical, and genetic differences in transsexual per- sons (Bosinski et al., 1997; Henningson et al., 2005; Kruiver et al., 2000; van Goozen, Slabbekoorn, Gooren, Sanders, & Cohen-Kettenis, 2002; Zhou, Hofman, Gooren, & Swaab, 1995). Nevertheless, little data exist supporting any single hypothesis. A few monozygotic (MZ) FTM and MTF twin pairs, concordant or discordant for transsexualism, have been reported (Segal, 2006). However, only a single MZ concor- dant FTM twin pair has been published (Sadeghi & Fakhrai, 2000). We report the second female MZ twin pair concordant for transsexualism. The sisters (BK and RK) grew up in a rural environment in Macedonia. They have three older brothers and three older H. Knoblauch () Vivantes Klinikum am Urban, Klinik f¨ ur Psychiatrie und Psychotherapie, Dieffenbachstr. 1, 10967 Berlin, Germany e-mail: Hans.Knoblauch@vivantes.de A. Busjahn HealthTwist GmbH, Berlin-Buch, Germany B. Wegener Vivantes Klinikum am Urban, Klinik f¨ ur Psychiatrie und Psychotherapie, Berlin, Germany sisters. Their father, a severe person, worked abroad and rarely saw his family (once every two years). The mother and an older brother assumed the parental roles for the girls. The mother was described as a “good mother” who “man- aged” the entire family. BK, the older twin sister, described herself as more reserved compared to the younger sister, RK, who described herself as livelier. BK was more frequently ill until the third year of life. RK reached her developmental milestones (walking and talking) around four months earlier than BK. RK had several accidents in childhood and adoles- cence; one accident was followed by a cerebral concussion. In contrast, the similarities between the girls were striking. Both sisters preferred to play with boys rather than girls since age six years. They tried to imitate all the things that boys did. They played soccer and other boys’ games. Both rejected dolls and girls’ clothing. Both twins preferred short hair and forced their mother to cut their hair short from age six years. In school, both sisters preferred sports, music, and history. At school, they were asked if they were boys or girls by the teachers and responded that they were boys. Both reported many conflicts with other children. With the onset of menar- che and breast development, they began to use bandages to flatten their breasts. They had not been sexually educated by their parents and prayed each evening to become and grow up as boys. The sisters began hating their bodies and stopped playing soccer. After the 8th grade, the twins moved to Germany to live with their father because of the growing social and fam- ily pressure related to their gender identity problems. The father forced his daughters to visit a school with mainly girls; however, both were treated as boys. Considerable con- flicts with the father developed. After the twins were legally independent, the twins moved to Berlin and decided to un- dergo a sex-change operation at age 25 years. Both twins de- nied heterosexual or homosexual contact and both were not Springer