Rs1801133

C677T or rs1801133 is a genetic variation—a single nucleotide polymorphism —in the MTHFR gene.
Among Americans the frequency of T-homozygosity ranges from 1% or less among people of sub-Saharan African descent to 20% or more among Italians and Hispanics.
It has been related to

In 2000 association studies on oral clefts, Down syndrome, and fetal [anticonvulsant syndrome] were either unreplicated or had yielded conflicting results.

Related genetic variants

is a SNP in the same gene.
Studies have investigated the combined effect of C677T and A1298C.