Our work was thus aimed at investigating, by means of direct co-culture, the interactions between a line of fluorescently tagged bone metastatic human breast cancer cells, MDA-MB231-BO-KL (BOKL) and a bone microenvironment represented by osteo-differentiated primary human bone marrow stromal cells (BMSCs), in terms of modifications on RANKL/RANK/OPG expression, cell proliferation and migration. The gene discussed is BOK; the disease is breast cancer.