The brain is the most sophisticated and intricate organ in the body. Billions of neurons interconnect and form distinct regions which process different neural activities. The development of the brain during pregnancy and early post-natal life is extremely sensitive, complex and crucial to proper function over the life of a person. This is the most plastic time of the brain. It is changing constantly and reacting to the different stimuli encountered by the individual. The lack of a particular stimulus can have a profound effect on the later structure and function of the brain. For example, if a newborn mouse has an eye covered so it receives no light, visual cortex, where normally processes binocular visual stimuli, develops to process visual stimuli only from the open eye. This cannot be altered later on even when both eyes are opened; the mouse remains weak in one eye despite there being nothing wrong with the eye itself. Studying this early time period of brain development presents many problems. Investigation is hampered by the difficulty in accessing and manipulating the brain as well as the huge variety of factors that contribute to brain development. Currently, most work is conducted in rodents, primarily because there are a large range of genetic tools available. This is useful to an extent and has demonstrated key findings that appear to be relevant to most mammalian species. However, the human brain is quite different to the mouse brain. It has adapted to very different tasks required of mice compared to humans and therefore there is a knowledge gap to bridge in this area. In addition to this, examination of global gene expression in the brain has only truly become viable in the last 10 years. The same can also be said of the ability to analyse the development process at a biochemical level. Dr Tomomi Shimogori of the RIKEN Center for Brain Science, Japan, has been tackling these difficulties through her work on the molecular mechanisms of brain development. She has worked on rodents, but is now developing a model in the common marmoset based around the creation of a gene atlas. Working on the primate should help fill in the gap between rodent and human. Shimogori explains why the marmoset was chosen: ‘One of the biggest advantages of using marmosets as a model animal is that many of its behaviours share similarities with human behaviours, and thus has potential for use in understanding the underlying mechanisms of human brain function and mental disease.