GDNF and brain neoplasm: It is noteworthy that none of the studied tumor spheroids reacted to HA by the altered expression of the most crucial markers in brain tumor pathogenesis, for example, Hif1a and Pdgfra, which control tumor growth in normoxia or mild hypoxia [49], the tumor suppressor Tp53 [50], or the oncogenes Melk and Gdnf [51].