This thesis examines the role of the guanine nucleotide exchange factor PREX2 in breast cancer. Analysis of human breast cancer patient databases demonstrates that mammary tumours express high levels of PREX2 compared to normal breast tissues, and high PREX2 expression correlates with breast cancer metastasis. Using breast cancer cell line models, this thesis shows that PREX2 expression results in increased cell growth, survival, metastasis and malignant transformation. Expression of PREX2 or a PREX2 breast cancer mutant also alters actin polymerisation and breast cancer cell morphology. Therefore, PREX2 may serve as a potential therapeutic target in future breast cancer treatments.