PTPRZ1 and glioblastoma: Parental C6 and U251 glioblastoma cells readily formed spheres in serum-free medium supplemented with EGF and FGF (sphere culture conditions) as previously described29, whereas RZ-KD#2 (a stable Ptprz-knockdown clone of C6) and RZ1-KD#5U cells (a stable PTPRZ1-knockdown clone of U251) did not (Fig. 1A, CSC).