PAX3 and spina bifida: Using animal models such as mice, chicks, Xenopus, and zebrafish, researchers have successfully constructed many NTD models to study the underlying mechanisms of failed neural tube closure.[33] Genetically tractable mice are widely used via chemical induction (e.g., retinoic acid exposure, folic acid antagonists, valproic acid, and hyperglycemia) or gene knockout (e.g., Pax3 and Sox2 mutations) to mimic human NTDs, such as spina bifida.[34] DHFR is the key enzyme involved in folate metabolism.