Gfi1 loss of function predisposes to leukemia. (A) Methylcellulose colony-forming assay with serial replating of wild-type and Gfi1^−/− (Gfi1^{Δex4-5/Δex4-5}) Lin⁻ bone marrow cells. (B) Methylcellulose colony-forming assay with serial replating of sorted Lin⁻ wild-type bone marrow cells transduced with retrovirus vectors expressing Gfi1 (Gfi1 WT) or an empty vector control (Vector). (C) Methylcellulose colony-forming assay with serial replating of sorted wild-type bone marrow cells transduced with retrovirus vectors expressing Gfi1 dominant-negative mutants (P2A or N382S) or an empty vector control (Vector). (D) Methylcellulose colony-forming assay with serial replating of Rosa-CreER^T2+Gfi1^{fex4-5 fex4-5} sorted GMPs with or without tamoxifen (OHT) added in vitro to activate the CreER^T2 protein and delete floxed Gfi1 alleles. (E) Survival curve of Mx1Cre⁺K-ras ^lslG12D (n = 7) or Mx1Cre⁺K-ras ^lslG12D Gfi1^{fex4-5/fex4-5} (n = 7) mice beginning at time of pIpC injections. (F) Photomicrographs (40 ×) of peripheral blood smears from animals upon humane killing from panel E with insets showing myeloid forms and blasts. (G) Peripheral white blood cell (WBC) counts from mice in panel E. (H) Peripheral blood myeloblast counts from mice in panel E. (I) Photomicrographs (5 ×) of H&E-stained splenic tissue from mice in panel E (primary) or recipients of 10⁵ spleen cells from moribund Mx1Cre⁺K-ras ^lslG12D Gfi1^{fex4-5/fex4-5} animals in panel E (secondary). Error bars indicate SEM. **P ≤ .01, ***P ≤ .001.