Although some studies have evaluated the association between MTHFR C677T polymorphism and CIN in populations exposed to different levels of FA [13], [17], [21], a formal interaction between MTHFR C677T polymorphism and circulating concentrations of folate on the risk of CIN in the US post-FA fortification era has not been reported. Here, MTHFR is linked to cervical squamous intraepithelial neoplasia.