Innate behaviours are well studied in many animals and yet little is known regarding how these behaviours are encoded via neural circuitry and how genes regulate their development. A number of “master switch” genes have been defined that specify (i.e. are necessary and sufficient for) developmental processes, e.g. the eyeless gene, which regulates the differentiation of multiple cell types to direct eye formation, even when expressed ectopically (1). Less well defined however is if, and how, master regulator genes are able to direct innate behaviour. Fruitless (fru) has been suggested to be one such “master switch” gene as it has been shown to specify male courtship behaviour in Drosophila (2). It is thought that fru acts by specifying sex-specific neural circuits within the CNS, which allow a particular, stereotyped behavioural response (3), however it is unclear at a molecular level, how this occurs. This project will explore, using fru as a model, how a single gene can direct the specification of neural circuitry underlying a given behaviour (Drosophila courtship). Understanding the molecular cascades directed by fru will not only allow an understanding of how genes control courtship behaviour, but more broadly suggest mechanisms by which the construction and coordination of neural circuits can be specified by genetic switches.