Rs1801133

C677T or rs1801133 is a genetic variation—a single nucleotide polymorphism (SNP)—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
 * schizophrenia
 * Alzheimer's disease
 * depression
 * autism
 * spina bifida.

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

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