MTHFR and Dravet syndrome: There is also evidence that genetic factors predisposing to chromosome 21 non-disjunction in maternal meiosis I might be different from those predisposing to errors occurring at maternal meiosis II.[35] It is therefore time to perform an updated systematic review and meta-analysis of the literature concerning the association of the polymorphisms C677T and A1298C of the maternal MTHFR gene with DS risk in attempt to address all these points.