ACKR3 and cancer: Although there have been contradictory reports where ACKR3 enhances CXCR4-mediated metastasis and others where it inhibits CXCR4 (refs 5, 7), emerging evidence suggests that it can signal through β-arrestin to ERK1/2, AKT and other pathways to promote tumor migration and survival as well as the survival and self-renewal of cancer stem cells8.