Abstract: Neural tube defects (NTDs) are deleterious birth defects that result in herniation and exposure of nervous tissue during embryogenesis when the neural tube fails to close. Folate deficiency has shown a high association to the development of NTDs and 70% of all NTDs are folate responsive. However, the mechanism of this rescue effect remains to be elucidated. The tumor suppressor protein p53 plays an important role in development. Studies in vitro demonstrate interactions between p53 and folate one carbon metabolism, specifically the de novo thymidylate (dTMP) biosynthesis pathway. The Stover laboratory has demonstrated that impairments in the de novo dTMP biosynthesis pathway ultimately leads to an increase in NTDs. Previous studies have demonstrated impaired de novo dTMP biosynthesis leads to uracil accumulation in DNA and genomic instability and another paper has shown that uracil accumulation in DNA leads to p53-mediated apoptosis. Compiling this information, a proposed mechanism of the development of folate deficient NTDs has been developed and will be examined. Specifically, this proposal aims to investigate the role of p53 in folate deficiency and how it may affect genomic instability and NTD incidence. The first part of the proposal will determine the effect folate deficiency has on the incidence of NTDs in the absence of p53 and whether p53 null NTDs are folate responsive. These results will illustrate the in vivo interaction between p53 and folate one carbon metabolism in the development of neural tube defects. In this aim, embryos will be phenotyped for NTDs. The second part of the proposal will determine the effect folate deficiency has in p53 null embryos in terms of genomic instability. This question will be answered by measuring de novo dTMP synthesis, uracil in DNA, and DNA damage with the biomarker ?H2AX. This proposal will uncover the interaction between p53 and folate deficiency in regards to de novo thymidylate synthesis, uracil in DNA, and genomic instability. Most importantly, this project may elucidate a possible mechanism for the development of genomic instability and NTDs in embryos from maternal folate deficient diets.