Impaired CD8⁺ T-cell development in Stat5a/b^f/flck-cre mice. (A) Splenic cells of 3 individual Stat5a/b^f/flck-cre (nos. 1-3) and 2 Stat5a/b^f/f (nos. 4-5) mice were magnetically sorted for Thy1.2⁺ cells, and Stat5a/b expression was assessed by Western blot analysis. (B) Representative flow cytometric profile of CD4⁺, CD8⁺, and CD4⁺/CD8y cells in thymus, blood, spleen, and lymph nodes of Stat5a/b^f/flck-cre mice and Stat5a/b^f/f controls. Asterisks denote significant difference as determined by a paired t test. (C) Analysis of transcriptional expression of pim-1, CIS, and cyclin D2 genes by semiquantitative RT-PCR. Splenic T cells were stimulated for 4 hours with α-CD3 (1 μg/mL) and human IL-2 (hIL-2; 1000 U/mL) to induce Stat5a/b target gene transcription. (D) Schematic model for the role of Stat5a/b in T-cell developmental choices. Stages affected in Stat5a/b^null/null survivor mice and/or Stat5a/b^f/flck-cre mice are indicated. The time point of Cre-recombinase activation under the control of the distal lck promoter is also depicted.