This is a mentored research scientist development award to support an intensive, supervised career development experience in the biomedical, behavioral, or clinical sciences leading to research independence. The trainee will study developmental mechanisms in the autism-related disorder Turner syndrome, a well-defined genetic disorder resulting from the partial or complete loss of one of the sex chromosomes. Currently, very little is known about sexual dimorphism in early development, and it is unclear why autism is more prevalent in males than females, though researchers have hypothesized X- chromosome effects and early exposure to gonadal hormones may play a causal role. In this longitudinal study, brain imaging techniques like magnetic resonance imaging (MRI) will be used to characterize brain development in typically developing children and children with Turner syndrome followed from birth to 2 years of age. Results will provide new information on brain-specific sex differences in the developing brain and a better understanding of brain abnormalities in Turner syndrome.