ABSTRACT Mouse extraembryonic endoderm stem (XEN) cell lines can be derived from preimplantation embryos (pre-XEN) and postimplantation embryos (post-XEN). XEN cells share a gene expression profile and cell lineage potential with primitive endoderm (PrE) blastocysts. However, the cellular origin of XEN cells in embryos remains unclear. Here, we report that post-XEN cell lines are derived both from the extraembryonic endoderm and epiblasts of postimplantation embryos and that pre-XEN cell lines are derived both from PrE and epiblasts of blastocysts. Our strategy consisted of deriving post-XEN cells from clumps of epiblasts, parietal endoderm (PE) and visceral endoderm (VE) and deriving pre-XEN cell lines from single PrE and single epiblasts of blastocysts. Thus, XEN cell lines in the mouse embryo originate not only from PrE and PrE-derived lineages but also from epiblast and epiblast-derived lineages of blastocysts and postimplantation embryos.