Dietary deoxyuridine and uridine have been shown to have opposing effects on neural tube defect (NTD) incidence in the serine hydroxymethyltransferase 1 (Shmt1+/–) mouse model of folate-responsive NTDs, which are mediated by changes in de novo thymidylate biosynthesis. Here, SHMT1 is linked to isolated spina bifida.