In our resected mouse mammary tumors, the CXCR4:GRK3 expression ratio was stable even after 6 weeks of tumor implantation and distant metastasis (S2 Fig), suggesting that the CXCR4:GRK3 ratio is preserved and could be a useful prognostic indicator at extended times during the disease course. The gene discussed is GRK3; the disease is breast cancer.