The tumors showed no overt differences in size or differentiation from teratocarcinomas generated by Esrrbfl/fl cells. Most significantly, the embryonic identity and developmental potential of Esrrb null cells was confirmed by integration into the inner cell mass after aggregation with morulae (Figures 5B and 5C) and widespread contribution to the midgestation embryo after blastocyst injection (Figure 5D; Figure S5). This evidence concerns the gene ESRRB and teratocarcinoma.