SOX2 and neoplasm: In addition, we were surprised to find that circPTN was downregulated in serum-induced glioma stem cell (S15) compared with non-serum cultured glioma stem cell (G15), and we observed that circPTN promoted G15 to form tumor spheres and express stemness markers such as SOX2, SOX9, CD133, and Nestin via the sponging of miR-145-5p.