IL-15 induces sustained STAT5 phosphorylation in SOCS1-deficient CD8⁺ SP thymocytes. Total thymocytes from 6-week-old SOCS1^-/-IFNγ^-/- mice and their littermate controls were stimulated with 40 ng mL^-1 IL-15 or a 1:20 dilution of IL-7-containing CS for 30 minutes. An aliquot of 2 × 10⁶ cells was fixed immediately after stimulation. The remaining cells were washed twice and cultured without cytokines. At indicated time points, aliquots of cells were fixed, permeabilized, and stained for intracellular phospho-STAT5 followed by CD4 and CD8 surface staining. Phospho-STAT5 levels in gated CD8⁺ and CD4⁺ thymocytes are shown in histograms. Histograms of the stimulated cells are shaded gray, and unfilled line histograms represent the unstimulated cells. Results shown are representative of at least 4 animals per group from 2 independent experiments.