RAC1 and breast cancer: In this study, we demonstrate that the Dedicator of Cytokinesis 4 (DOCK4), a GEF implicated in RAC1 activation downstream of EGFR signalling18, is necessary for induction of a cancer cell mesenchymal-like morphology compatible with intercalation of breast cancer cancer cells into brain endothelium in vitro, and brain metastatic extravasation in vivo.