Fig 2 Genotype, concentration, and risk model components used to assess association between red blood cell (RBC) folate concentration and risk of neural tube defects among Community Intervention Project participants. Genotype model: MTHFR genotype frequencies have been shown to be different in northern (36% TT in Folic Acid Dosing Trial) and southern Chinese populations (8% TT).^{20 25 26} Each person was assigned a probability of having a specific genotype. RBC folate concentration model: RBC folate concentration at time of neural tube closure was estimated on basis of region of residence (northern v southern), probable genotype, and folic acid supplementation history. Substantial regional differences in diet have been previously described, with northern regions having less natural folate intake and lower RBC folate concentrations.^{1 29 30 31} As a result, the concentration model included a term reflecting regional differences in baseline RBC concentration. Neural tube defect risk model: risk of neural tube defects associated with RBC folate concentrations was estimated for Community Intervention Project participants on basis of observed neural tube defect occurrence and estimated RBC folate concentration