The etiology of gender dysphoria (GD) is not fully known. In recent years, there are studies suggesting that the development of GD is related to some biological mechanisms. The hypothesis conveying that there is a biological background in the development of GD emphasize the importance of exposure to sex hormones in prenatal period. According to this hypothesis, while prenatal exposure to sex hormones causes development of male identity, nonexposure results with female identity. This hypothesis is supported with determination of the relationship between prenatal exposure to testosterone and masculine behaviors, in complete androgen insensitivity syndrome mostly development of female identity, and presence of near values of 2nd and 4th finger ratio in individuals with GD and the individuals with the sex they want to pass. In this article, one of the twins is male and the other one is female-to-male GD case was discussed in terms of the roles of prenatal exposure to sex hormones in the etiology of GD.