SPP1 and neoplasm: By analyzing the most common tumor microenvironmental cytokines, we observed that basal BCSCs, as well as cancer-associated fibroblasts (CAFs) isolated from a basal cancer sample, secrete hepatocyte growth factor (HGF), stromal cell-derived factor 1 (SDF-1) and osteopontin (OPN) in the medium (Figure 4A).