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. This evidence concerns the gene CXCL12 and prostate carcinoma.