Deletion of the gene encoding NAD-dependent methylenetetrahydrofolate dehydrogenase-cyclohydrolase (NMDMC) in mice was demonstrated previously to result in failure to establish definitive erythropoiesis in the developing liver. We examined the expression pattern of nmdmc to look for evidence that would support a tissue specific role for this activity. However, whole mount in situ hybridization revealed ubiquitous expression of nmdmc in the tissues of E9.5 and E10.5 embryos suggesting a broader role. Analysis of chimeras demonstrated that nmdmc-/- cells can survive in liver and other tissues of chimeras establishing that the null defect can be rescued by metabolites supplied by surrounding normal cells. Both the expression pattern and metabolite rescue support the proposal that mitochondrial NMDMC provides one-carbon units for purine synthesis during embryogenesis. The elevated expression of NMDMC in tumour cells, but not in surrounding normal cells, is predicted to result in significant differences in folate-mediated support for purine synthesis in the two cell types.