The purpose of this project is to better understand the influences underlying gender differences in the brain using Four Core Genotype mice. Four Core Genotype mice are transgenic mice in which the SRY gene has been translocated from the Y chromosome to another location. This enables separation of the genetic sex and gonadal sex. For example, there are female mice based on sexual organs but their chromosomes are XY(UY- in mice). This allows us to determine whether sexual differentiation in the brain is due to genes or hormones. In this project, I looked at a sexually dimorphic area of the brain, the Bed Nucleus of the Stria Terminalis (BNST), which is twice as large in males than in females. I hypothesize that both chromosomes and gonadal hormones play a part in sexually differentiating the brain including the BNST and thus I predict that the size of the BNST will be the same in XX males (UUSRY) and XY females (UY-). I measured the BNST from five XX female (UU) and five XY male(UY-SRY) four core genotype mouse brains and confirmed that the BNST is larger in males than in females, as it is in normal mice (p= .057). I processed and measured the size of the BNST in ten brains of XX males and XY females to see if the size of the BNST matches the chromosomes or the gonads. The results had a trend in the data that suggested chromosomes play more of an effect on sexual differentiation of the BNST. The overall goal of this project is to contribute to research examining the causes of gender identity in humans by relating this work to other works in the field.