I have found some conflicting information regarding the SNPs that are responsible for the lesser functional ability of MTHFR. I had Promethease analyze my 23andme SNPs and it said that I have the CC version of both rs1801133 and rs1801131. The analysis said that I "have trouble metabolizing common vitamin pills." However, I found this article with the following: The most-studied polymorphism is the 677T)C (rs1801133) in exon 5 of the 5,10-methylenetetrahydrofolate reductase (MTHFR [MIM 607093]) gene. The 677T variant results in an thermolabile enzyme that is less effective in the conversion of 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate.20 Accordingly, individuals with the 677T variant have higher homocysteine concentrations as well as lower genomic DNA methylation. 21,22 So it is saying that the TT version is the ineffective one. The article is from: http://www.sph.umich.edu/csg/abecasis/publications/pdf/Am.J.Hum.Genet.vol.84-pp.477.pdf My understanding is that the C replacing T version is the ineffective one. Does anyone have any idea which is right?