When advanced thyroid cancer patients with multiple metastasis do not secrete Tg or Tg is undetectable because of anti-TgAb, serial follow-up based on serum Cyfra 21.1 levels might be used as an alternative biomarker for disease monitoring comparable to serum Tg, especially in BRAF-mutated thyroid cancer. The gene discussed is BRAF; the disease is thyroid gland carcinoma.