T-lymphoblastic lymphoma is of ES-cell origin in Runx1^lacZ/lacZ chimeras. (A) Immunohistochemistry using an anti–β-galactosidase antibody demonstrates that the malignant cells in the spleens of LZD nos. 2 through 5 express β-galactosidase and are therefore derived from Runx1^lacZ/lacZ ES cells. The normal splenocytes from LZD no. 6 (which did not develop lymphoma) serve as negative controls. (B) Southern blot analysis detects presence of only the Runx1-LacZ knock-in allele in lymphoma-effaced bone marrow from LZD nos. 2 through 4 but only wild-type Runx1 allele from normal bone marrow of LZD no. 6. (C) Genomic PCR using LacZ gene primers and DNA from sorted thymic cells of a Runx1^lacZ/lacZ chimera. Lane 1, CD8⁺; lane 2, CD8⁺/CD4⁺; lane 3, CD4^-/CD8^-; lane 4, CD4⁺; lane 5, total thymus; lane 6, total thymus from a wild-type mouse; lane 7, no DNA. (D) Genomic PCR using LacZ gene primers and DNA from sorted bone marrow cells of a Runx1^lacZ/lacZ chimera. Lane 1, c-kit⁺/lin^-; lane 2, c-kit⁺/lin⁺; lane 3, c-kit^-/lin⁺; lane 4, c-kit^-/lin^-; lane 5, total bone marrow.