Considering the importance of GRK3 regulation of breast cancer migration, invasion, and metastasis described in this manuscript, understanding which tumors have more dysregulated signaling through CXCL12/CXCR4 could be used as a biomarker to predict which patients might respond better to CXCR4 antagonism or as an alternative, targeted therapy directed specifically at GRKs. The gene discussed is CXCL12; the disease is breast cancer.