The three TET enzymes actively demethylate DNA and mice deficient of the three enzymes die shortly after gastrulation. We revisited this mutant phenotype using temporal single-cell atlases from embryos with partial or complete mutant contributions. Strikingly, when developing within a wild-type embryo, Tet-mutant cells retain near-complete differentiation potential, whereas full-embryo mutants fail to generate ectoderm and most embryonic mesoderm lineages.