However, when authors searched for the simultaneous presence of more than two risk alleles, they often reported increased maternal risk for a DS birth with the increasing number of variants in folate pathway genes carried by the mother: for example, da Silva et al. (2005) observed that MDS tend to present more variant alleles than control mothers among MTHFR 677C>T, MTHFR 1298A>C, MTR 2756A>G, MTRR 66A>G, and CBS 844ins68 polymorphisms, and the presence of each uncommon allele was associated with an increment of 25.9% in the risk of a DS pregnancy (da Silva et al., 2005). The gene discussed is CBS; the disease is Dravet syndrome.