MTHFR and Alzheimer disease: GIS maps were further generated and demonstrated the potential associations between the geographic pattern of MTHFR A1298C polymorphism and AD risk, as well as their impact on AD risk among studies conducted in India, Japan, Tunisia, Poland, Germany, and Brazil (see Figure S7 for combined CC and AC genotypes, Figure S8 for CC homozygous genotype, and Figure S9 for AC heterozygous genotype).