Conditional SCL activation results in decreased apoptosis and decreased cycling of CD8⁺TCRβ^low thymocytes. (A) The proportion of dead thymocytes was examined by propidium iodide (PI) staining. Wild-type, n = 4; lck-ER^T2-SCL, n = 4; TAM feed for 12 weeks. Error bars represent means plus or minus SD. (B) Early apoptosis was investigated by the loss of the mitochondrial membrane potential (Δψ), depicted by the loss of JC-1 red fluorescence. The proportion of JC-1–negative (early apoptotic) cells was established within forward and side scatter gated DP (CD4⁺CD8⁺, double-positive) and CD8 single-positive (SP) thymocytes. Wild-type, n = 3; lck-ER^T2-SCL, n = 3; TAM feed for 11 weeks. (C) The proportion of apoptotic thymocytes (annexin V⁺/ PI⁻) was determined within forward and side scatter gated cells. Representative plots of indicated populations are shown. Wild-type, n = 3; lck-ER^T2-SCL, n = 3; TAM feed for 7 weeks. (D) Quantification of annexin V/ PI flow cytometric analysis. Bar graphs represent the proportions (± SD) of apoptotic (annexin V⁺/ PI⁻) cells within thymic subsets. Error bars represent means plus or minus SD. (E) The proliferation of thymocytes was measured by in vivo BrdU incorporation. Mice (treated with TAM for 5 weeks) were exposed to BrdU for 36 hours before thymus harvest. Representative plots are shown. (F) Quantification of BrdU flow cytometric analysis. Wild-type, n = 4; lck-ER^T2-SCL, n = 3; TAM feed for 5 weeks. Bars represent means plus or minus SD; *P < .05; **P < .001. SP indicates single-positive cells.