The secretory small GTPase Rab27B masters the release of pro-invasive growth regulators in poor prognosis ER-positive breast cancer. In this research project we characterize the Rab27B-vesicles, identify the effector used to traffic Rab27B-vesicles towards the cell periphery, study effector involvement in invasive tumor growth, and translate the experimental data to the clinic by validating Rab27B effector status in primary breast cancer.