Fig. 9. Mixl1^–/– ES cells (identified by the lack of lacZ activity) display reduced potency to colonize the embryonic endoderm. (A) A chimera with more than 90% contribution of Mixl1^–/– ES cells recapitulates the phenotype of Mixl1^–/– embryo. (B) A chimera with low Mixl1^–/– ES cell contribution shows widespread distribution of the mutant cells in the embryonic tissue (C, plane of sectioning indicated in B), but poor contribution to the endoderm of the foregut (fg) and hindgut (hg). (D) A chimera with high mutant ES cell contribution shows extensive colonization of most embryonic tissues by the mutant cells, but sparse presence in the foregut endoderm (E) and complete absence from the hindgut endoderm (F). Planes of sectioning for E,F are indicated in D.