Reintroduction of Pbx1b in Pbx1^−/− ES cells restores B-cell and CLP development in Rag1-deficient blastocyst-complemented mice. (A) FACS analysis shows the presence of IgM⁺B220⁺ B cells and normal numbers of TCRαβ⁺ cells in the peripheral blood of mice chimeric for Pbx1b-rescued ES cells at levels comparable with Pbx1^+/+Rag1^−/− control mice. (B) FACS analysis shows that donor-derived (Ly9.1⁺) CLPs are present in Pbx1^−/−RRag1^−/− chimeras.