Since we have previously reported that CDK5 phosphorylates STAT3 at Ser727 site in prostate and medullary thyroid cancer cells [22,28,33], thus, we thought that STAT3 might be a potential downstream of CDK5 in GDNF-induced RET signaling in medullary thyroid cancer cell proliferation. This evidence concerns the gene GDNF and medullary thyroid gland carcinoma.