CXCL12 and glioblastoma: The chemo-attractant, stromal-derived factor-1α (SDF-1α)/ hemokine (C-X-C motif) ligand 12 (CXCL-12), the C-X-C receptor type 4 (CXCR-4), and the cysteine protease cathepsin K (cat K) are localized to GSC niches in GBM (Hira et al., 2015) and SDF-1α acting through its interactions with CXCR-4 and/or its second receptor CXCR-7 on GSCs facilitates the homing of GSCs to niches, while cat K, which is up-regulated by ROS (H2O2) (Tsai et al., 2014), can cleave and thereby inactivate SDF-1α and, in doing so, facilitate the migration of GSCs out of the niches.