TET1 and neural tube defect: Further backcrossing (≥13 generations) to create a highly inbred 129S6 congenic (129S6.Cg) strain reduced NTD rates in Tet1 KO to below 20% (Fig. 1E), suggesting that some genetic heterogeneity (in CD1 and 129S6.B6 incipient congenic) is necessary to promote the phenotypic expression of NTDs in Tet1 KO embryos.