RAC1 and breast cancer: In non-transformed cells, ECT2 regulates cytokinesis by regulating Rac1 activity.13–16ECT2 is frequently upregulated in various cancers such as ovarian,14 lung,17 and breast cancers.18 ECT2 promotes Rac1 activity and increases cell growth, invasion, and tumorigenicity.13,16 Here, we show that PRICKLE1 is associated with ECT2 to regulate Rac1 activity and that Prickle1 and Ect2 act synergistically during embryonic development.