Almost all tumours display increased activity of components of the phosphoinositide 3-kinase (PI3K) signalling pathway. In particular, activating mutations in the PI3K catalytic subunit, p110alpha, and loss of function mutations of the opposing PTEN phosphatase are amongst the most frequent alterations in human cancers. Our current work aims to understand how loss of PTEN function contributes to tumour development through two related projects. Firstly, we are attempting to address the significance of PTENs dual specificity for both phosphoinositide lipids and peptide substrates through the development and use of mutants that selectively metabolise either lipid or protein substrates. These data indicate a requirement for both activities in the regulation of invasion and proliferation in 3D matrigel culture models of glioblastoma. Secondly, we wish to determine the significance of PTEN in the generation of phosphoinositide gradients within epithelial cells, and see whether this localisation of PTEN and its substrates is important in maintaining epithelial tissue architecture.