Decades of research on neurodevelopmental disorders have focused on genetics. Although there has been significant progress, the aetiology of many neurodevelopmental disorders still remains unknown. Deciphering genetic sequences of the whole genome can identify disease-causing mutations in individuals. However, the same genetic sequences do not necessarily result in similar gene expression profiles, or the consequential biochemical profiles in every cell and in all individuals. In particular, studies have shown that differential biochemical profiles in males and females, possibly play a role in neurodevelopmental disorders being biased towards a different gender. Interestingly, autism spectrum disorder (ASD) is biased towards boys although it is not an X-linked disorder, whereas Rett syndrome, an ASD-related disorder where the disease-causing gene is located on the X-chromosome, is found almost exclusively in girls.