SOX2 and neoplasm: In parallel with the emerging function of SOX2 in various types of stem cells, and in particular in neural stem cells (see above, Section 3), the concept of “cancer stem cells” (CSC) or “tumor-initiating cells” (TIC) was emerging from studies on the cellular basis of tumorigenesis: tumors were found to be heterogeneous, and to contain a fraction of cells able to re-initiate tumor development following transplantation; these cells were resistant to most conventional chemotherapy, and were responsible for tumor relapse.