Cell proliferation and differentiation are tightly regulated processes required for the proper development of multi-cellular organisms. To understand the effects of cell proliferation on embryo patterning in mice, we inactivated Aurora A, a gene essential for completion of the cell cycle. We discovered that inhibiting cell proliferation leads to different outcomes depending on the tissue affected. If the epiblast, the embryonic component, is compromised, it leads to gastrulation failure. However, when Aurora A is inactivated in extra-embryonic tissues, mutant embryos fail to properly establish the anteroposterior axis. Ablation of Aurora A in the epiblast eventually leads to abnormal embryos composed solely of extra-embryonic tissues. We took advantage of this phenomenon to generate embryonic stem (ES) cell-derived mice. We successfully generated newborn pups using this epiblast ablation chimera strategy. Our results highlight the importance of coordinated cell proliferation events in embryo patterning. In addition, epiblast ablation chimeras provide a novel in vivo assay for pluripotency that is simpler and more amenable to use by stem cell researchers.