Cell invasion is a critical step in cancer metastasis. Rho family GTPases are key regulators of cell motility, regulating the form and function of the actin cytoskeleton. P-Rex1, a Rac guanine nucleotide exchange factor (GEF), was previously identified in this lab in a transcriptional screen for mediators of cell invasion. A number of GEFs have been shown to regulate the invasive phenotype of tumours and cancer cell lines, leading to the hypothesis that P-Rex1 is an invasion promoting gene. We show that overexpression of P-Rex1 induces dramatic morphological changes in fibroblasts characterised by extensive membrane ruffling and lamellipodia formation. P-Rex1 modulates the 2D migration of fibroblasts and potently stimulates their in vitro invasion. These phenotypes are dependent on Rac1, PI3 kinase and GPCR signalling and can be stimulated by growth factor mediated accumulation of PIP3. P-Rex1 expression is shown to be upregulated in a number of melanoma derived cell lines compared to normal human melanocytes. Ectopic overexpression of P-Rex1 enhances the membrane ruffling and invasion of melanoma derived cell lines, while in some cell lines, limiting its expression by RNAi reduces both serum stimulated ruffling and invasion. The P-Rex1-/- mouse is shown here to have a previously uncharacterised pigmentation phenotype, consistent with a defect in melanocyte precursor migration. This phenotype is not overcome by crossing on to a melanoma model driven by expression of activated N-RasQ61k and loss of the INK4a locus. While it seems that P-Rex1 deficiency does not affect melanoma initiation, studies are ongoing to determine whether it has a role in metastastic progression. Data also suggests that P-Rex1 has an unexpected role in lymphoma development. This work further characterises the regulation and function of P-Rex1, adding to our understanding of how RhoGTPase GEFs function in cancer.