MDA-MB-231 expressing green fluorescent protein (GFP) was a generous gift from Weinberg lab17 (link). Normal breast fibroblast (NBF) and cancer-associated fibroblast (CAF) were provided by Orimo lab and previously described by Kojima et al.24 (link) MCF-7 and normal human dermal fibroblast (NHDF) were purchased from ATCC. For in vivo studies, MCF-7 cells were engineered to express firefly luciferase with a lentivirus containing luciferase reporter (MCF-7/Luc) following a previous report67 (link). Primary human bone marrow mesenchymal stromal/stem cells (BMSCs) were derived from whole human bone marrow aspirates (Lonza)68 (link). BMSCs were expanded in alpha-modified Minimum Essential Medium Eagle (alpha-MEM), 20 mg L−1 gentamycin, 10% FBS, 2.5 ug L−1 rhFGF-basic (R&D Systems), 100 U ml−1 penicillin, and 100 mg ml−1 streptomycin. All other cells and co-cultures were grown in Dulbecco’s MEM (DMEM) supplemented with 10% FBS, 100 U ml−1 penicillin and 100 μg ml−1 streptomycin.