It also worth mentioning that, when comparing the neurite outgrowth enhanced by the glial cell line-derived neurotrophic factor (GDNF) and that enhanced by the secretome of 5% O2 cultures, there was no significant difference, while GDNF significantly enhanced neurite outgrowth of neuroblastoma cells compared to the secretomes of 3% O2 and 20% O2 cultures (Fig. 4c,d). This evidence concerns the gene GDNF and neuroblastoma.