However, erythropoietin and estradiol (in men) were present in glioblastoma cyst fluid at higher concentrations than in serum, suggesting formation of these hormones by tumor or brain tissue, which would explain why their levels did not correlate with the levels of serum proteins and which would be in agreement with previous studies showing erythropoietin and estradiol formation by glioblastoma [21, 46]; erythropoietin formation could be triggered by hypoxic conditions. This evidence concerns the gene EPO and glioblastoma.