The CXCL12/CXCR4 axis is a physiological mediator of bone marrow homing and quiescence of hematopoietic stem cells5 and also regulates the migration of breast and prostate cancer cells towards tissues with a high-CXCL12 expression including bone.6,7 Based on previous in vitro data showing that the CXCL12/CXCR4 axis regulates the osteotropism of NET cancer cells,12 we analysed the expression of CXCR4 on CTCs in a proportion of our patients and found that it tended to be higher in patients with bone metastases than in those without it. The gene discussed is CXCR4; the disease is prostate cancer.