Neural tube defects (NTD) are the most common and severe CNS malformations. NTD are complex traits, involving interactions between environmental agents and multiple predisposing genes. The molecular basis for NTD is not understood. We are examining the curly tail (CT) mouse, the mutant which most closely resembles human NTD in both its genetic complexity at least 3 genes are involved (Genomics 29, 719724, 1995) and phenotype: the sole defects are lumbosacral NTD and a rare `anencephaly equivalent' more common in females. We hypothesize that the CT mouse shares a molecular basis with common human NTD. We have mapped the gene of major effect ct, to distal chromosome 4, syntenic with human 1p3536 (Nature Genetics 6, 357362, 1994). We now show by haplotype analysis that ct is semidominant; 10% of affected offspring carry only a single copy of the mutant gene, preventing usual mapping approaches. Therefore, homozygosity mapping was used to indicate those regions within the ct locus that likely contain the ct gene. Five plausible candidate genes mapped outside these regions and were ruled out by this approach, including MacMARCKS (aka F52 and Mrp) whose product is involved in cell shape change and when deleted, yields a CTlike NTD. Some NTD are prevented by periconceptional folate. Thus, we examined Mthfr because it codes for an enzyme required for the regeneration of active folate and maps to human 1p3536. Mthfr is excluded because it maps 2cM outside the mouse ct locus. Moreover, we find that MTHFR is not linked to NTD in a British/Irish human population, consistent with our hypothesis. Continued genetic dissection of the ct locus will enable a positional cloning approach to the ct mutation. Such genetic analyses of appropriate mouse models may be needed to identify the genes responsible for complex traits in man. (Supported by HD28882, a Reynolds Rich Smith Fellowship and SPR/APS Student Research Program).