Vitamin D also upregulates several cell cycle checkpoint inhibitors, such as p21, p18, and p27.25, 26, 27 Telomerase upregulation is common in HNSCC, and vitamin D treatment can inhibit the expression of Telomerase Reverse Transcriptase (TERT) in vitro.27 The gene discussed is TERT; the disease is head and neck squamous cell carcinoma.