The overarching focus of my research program is identifying the mechanisms underlying individual differences in neural development and behaviour. To do this, my research program focuses on the most well-conserved individual differences among mammals - sex differences. The proposed research will evaluate where hormones are acting and how hormone action within identified sites of action influence the development of the socio-sexual' circuit in the brain. This work stems from the hypothesis that socio-sexual behaviours are mediated by hormone action in multiple neural and non-neural tissues, each having a distinct role in shaping the neural circuitry that underlies sex-typical behaviours. The short-term goals of my lab are to identify where in the brain and body hormones act to affect sex-typical behaviours, and investigate the interaction between regions within an established circuit in the brain (e.g., evaluate how the sexual differentiation of one brain region affects the development of other regions within the same circuit). To do so, we will begin with a mouse model that is insensitive to androgens, like testosterone, throughout the brain and body due to a mutation in the androgen receptor (AR) - the testicular feminization mutation (Tfm) mouse. These XY chromosomal males present as female-typical in their body, brain and behaviour. We will then restore functional AR in select structures along the "socio-sexual" neural pathway in Tfm mice using the Cre-loxP transgenic system. Specifically, we will cross Tfm mice with mice that have an AR transgene activated by Cre; distinct mouse Cre lines will drive AR expression exclusively in target regions in the brain as well as candidate peripheral targets associated with socio-sexual behaviours (e.g., vomeronasal organ and genitalia). The mouse models we create will be sex-reversed in the targeted region (i.e., male-typical) relative to all other aspects of the animal (i.e., other body and neural regions will remain female-typical). Aim 1 will delineate the sufficiency of AR in neural vs non-neural tissue for socio-sexual behaviour by restoring functional AR to either neural tissue alone or globally in a mouse model completely null for AR (i.e., Tfm mice). Aim 2 will identify how androgen action in selected neural regions affect the development of the target region itself, as well as the entirety of the socio-sexual' neural circuitry and behaviour. Aim 3 will assess the role of AR in selected tissues peripheral to the brain (i.e., genitalia and VNO) for the development of the socio-sexual' neural circuitry and behaviours. Together, the proposed research will indicate where and how hormones act to mediate the sexual differentiation of brain and behaviour. While the proposed research will focus on a given neural circuit, it serves as an excellent prototype for advancing our understanding of the mechanisms involved in the emergence of individual differences in brain and behaviour.