BDNF and breast cancer: NGF, BDNF and NT3 pathways represent survival and proliferation signals in CSCs, by activating the Son of Sevenless (Sos)-Ras-MAPK and Fibroblast growth factor receptor substrate 2 (Frs2)/Ankyrin Repeat-Rich Membrane Spanning (ARMS)-Crk pathways, leading to cAMP response element-binding protein (CREB) and NF-kB stimulation, and finally controlling key cellular check-points implicated in CSCs proliferation in glioma, HNSC, melanoma, and breast cancer [37].