CXCL12 and breast cancer: Both the ER- MDA-MB-231 and ER+ T47D BC cell lines can remain dormant in bone marrow stroma co-culture through a gap-junction-mediated transfer of miR-127, −197, −222 and −223, which target C-X-C motif chemokine ligand 12 (CXCL12) [83,84].