All neural cell types of the brain are derived from neural progenitors (Götz and Huttner, 2005). Like all stem cells, neural progenitors / stem cells (NP/NSC) need to balance their capacity to self-renew, in order to maintain a stem cell pool, whilst maintaining the capacity to differentiate into neural and glial lineages. To achieve this balance NPs need to interpret multiple external signals arising from the stem cell niche or systemic circulation, and respond in a temporally and anatomically appropriate manner. This process occurs during both embryonic development of the brain as well homeostasis of NSCs in specialized niches in the adult. One protein ideally placed to coordinate NP/NSC responses to multiple signals and mediate rapid and quantitative responses is the deubiquitylating enzyme, USP9X. As a post- translational modifier USP9X can rapidly alter substrate levels and intracellular localisation without the need for gene transcription and translation. The balance between DUB and E3 ubiquitin ligase activity tightly regulates substrate levels. In addition, USP9X is highly expressed in NP/NSC in vivo and can regulate their responses in vivo and in vitro. The overarching aim of this project was to investigate the role of USP9X, if any, in the regulation of NP/NSC proliferation and by extension its putative role in brain cancer as either an oncogene or tumour suppressor.