Due to the limited benefit that patients with tumors without MGMT promoter methylation derive from temozolomide, especially as first-line treatment for glioblastoma, the possibility of detecting MGMT promoter methylation in liquid biopsy has been extensively studied with the aim of avoiding aggressive treatments in patients who will likely not benefit, especially in biopsy-only, low performance status or elderly patients. The gene discussed is MGMT; the disease is glioblastoma.