TYMS and esophageal squamous cell carcinoma: Furthermore, regarding the risks of esophageal squamous cell carcinoma and gastric cardia adenocarcinoma, a significant association was detected between low serum folate levels (<3 ng/ml) and polymorphisms of thymidylate synthase, which also requires 5, 10-methylene-tetrahydrofolate as the methyl donor [23].